Hardcoding Subs with ffmpeg?

In order to hardcode subtitles, your ffmpeg version needs to be compiled with libass support. Download a static build from https://ffmpeg.org/download.html, and then use:

ffmpeg -i input.wmv -vf "subtitles=sub.srt" \
  -c:v libx264 -crf 20 \
  -c:a aac -b:a 192k
  output.mp4

Here, you set the source of the subtitles filter to the file containing the subtitles. If you want to burn the subtitles of the input video file itself, replace sub.srt with the name of the input video file.

You can change the CRF value to change the video quality/size. More info on CRF here.

More info on burning subtitles can be found on the FFmpeg Wiki.

Note that I replaced the WMV/WMA output from the original question with H.264 and AAC. ffmpeg doesn't have an encoder for recent versions of WMV, so your output would have looked quite bad. And since you're hardcoding the subtitles, you have to re-encode the video anyway.

In the OP's question, there was a typo:

[AVFilterGraph @ 0000000002c5b4a0] No such filter: 'subtitle'
Error opening filters!

The filter is called subtitles with an s. Also, the ffmpeg version was not compiled with --enable-libass, so it could not be used to hardcode subs.

I have successfully tested this command:

ffmpeg -i input.mp4 -vf "subtitles=subtitle.srt" output.mp4

Note: If the output says something about No usable fontconfig configuration file found, using fallback, you must create inside ffmpeg's bin folder a new folder named fonts with a file inside (fonts.conf) that specifies where ffmpeg must look for fonts.

I have used this file in order for it to work (tested on Windows):

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- /etc/fonts/fonts.conf file to configure system font access -->
<fontconfig>

<!-- Font directory list -->

   <dir>./</dir>
   <dir>../fonts</dir>
   <dir>./fonts</dir>
   <dir>~/.fonts</dir>
   <dir>WINDOWSFONTDIR</dir>

<!--
  Accept deprecated 'mono' alias, replacing it with 'monospace'
-->
   <match target="pattern">
      <test qual="any" name="family">
         <string>mono</string>
      </test>
      <edit name="family" mode="assign">
         <string>monospace</string>
      </edit>
   </match>

<!--
  Accept alternate 'sans serif' spelling, replacing it with 'sans-serif'
-->
   <match target="pattern">
      <test qual="any" name="family">
         <string>sans serif</string>
      </test>
      <edit name="family" mode="assign">
         <string>sans-serif</string>
      </edit>
   </match>

<!--
  Accept deprecated 'sans' alias, replacing it with 'sans-serif'
-->
   <match target="pattern">
      <test qual="any" name="family">
         <string>sans</string>
      </test>
      <edit name="family" mode="assign">
         <string>sans-serif</string>
      </edit>
   </match>

<!--
  Load local system customization file
-->
   <include ignore_missing="yes">conf.d</include>

<!-- Font cache directory list -->

   <cachedir>WINDOWSTEMPDIR_FONTCONFIG_CACHE</cachedir>
   <cachedir>~/.fontconfig</cachedir>

   <config>
<!--
  These are the default Unicode chars that are expected to be blank
  in fonts.  All other blank chars are assumed to be broken and
  won't appear in the resulting charsets
 -->
      <blank>
         <int>0x0020</int>   <!-- SPACE -->
         <int>0x00A0</int>   <!-- NO-BREAK SPACE -->
         <int>0x00AD</int>   <!-- SOFT HYPHEN -->
         <int>0x034F</int>   <!-- COMBINING GRAPHEME JOINER -->
         <int>0x0600</int>   <!-- ARABIC NUMBER SIGN -->
         <int>0x0601</int>   <!-- ARABIC SIGN SANAH -->
         <int>0x0602</int>   <!-- ARABIC FOOTNOTE MARKER -->
         <int>0x0603</int>   <!-- ARABIC SIGN SAFHA -->
         <int>0x06DD</int>   <!-- ARABIC END OF AYAH -->
         <int>0x070F</int>   <!-- SYRIAC ABBREVIATION MARK -->
         <int>0x115F</int>   <!-- HANGUL CHOSEONG FILLER -->
         <int>0x1160</int>   <!-- HANGUL JUNGSEONG FILLER -->
         <int>0x1680</int>   <!-- OGHAM SPACE MARK -->
         <int>0x17B4</int>   <!-- KHMER VOWEL INHERENT AQ -->
         <int>0x17B5</int>   <!-- KHMER VOWEL INHERENT AA -->
         <int>0x180E</int>   <!-- MONGOLIAN VOWEL SEPARATOR -->
         <int>0x2000</int>   <!-- EN QUAD -->
         <int>0x2001</int>   <!-- EM QUAD -->
         <int>0x2002</int>   <!-- EN SPACE -->
         <int>0x2003</int>   <!-- EM SPACE -->
         <int>0x2004</int>   <!-- THREE-PER-EM SPACE -->
         <int>0x2005</int>   <!-- FOUR-PER-EM SPACE -->
         <int>0x2006</int>   <!-- SIX-PER-EM SPACE -->
         <int>0x2007</int>   <!-- FIGURE SPACE -->
         <int>0x2008</int>   <!-- PUNCTUATION SPACE -->
         <int>0x2009</int>   <!-- THIN SPACE -->
         <int>0x200A</int>   <!-- HAIR SPACE -->
         <int>0x200B</int>   <!-- ZERO WIDTH SPACE -->
         <int>0x200C</int>   <!-- ZERO WIDTH NON-JOINER -->
         <int>0x200D</int>   <!-- ZERO WIDTH JOINER -->
         <int>0x200E</int>   <!-- LEFT-TO-RIGHT MARK -->
         <int>0x200F</int>   <!-- RIGHT-TO-LEFT MARK -->
         <int>0x2028</int>   <!-- LINE SEPARATOR -->
         <int>0x2029</int>   <!-- PARAGRAPH SEPARATOR -->
         <int>0x202A</int>   <!-- LEFT-TO-RIGHT EMBEDDING -->
         <int>0x202B</int>   <!-- RIGHT-TO-LEFT EMBEDDING -->
         <int>0x202C</int>   <!-- POP DIRECTIONAL FORMATTING -->
         <int>0x202D</int>   <!-- LEFT-TO-RIGHT OVERRIDE -->
         <int>0x202E</int>   <!-- RIGHT-TO-LEFT OVERRIDE -->
         <int>0x202F</int>   <!-- NARROW NO-BREAK SPACE -->
         <int>0x205F</int>   <!-- MEDIUM MATHEMATICAL SPACE -->
         <int>0x2060</int>   <!-- WORD JOINER -->
         <int>0x2061</int>   <!-- FUNCTION APPLICATION -->
         <int>0x2062</int>   <!-- INVISIBLE TIMES -->
         <int>0x2063</int>   <!-- INVISIBLE SEPARATOR -->
         <int>0x206A</int>   <!-- INHIBIT SYMMETRIC SWAPPING -->
         <int>0x206B</int>   <!-- ACTIVATE SYMMETRIC SWAPPING -->
         <int>0x206C</int>   <!-- INHIBIT ARABIC FORM SHAPING -->
         <int>0x206D</int>   <!-- ACTIVATE ARABIC FORM SHAPING -->
         <int>0x206E</int>   <!-- NATIONAL DIGIT SHAPES -->
         <int>0x206F</int>   <!-- NOMINAL DIGIT SHAPES -->
         <int>0x2800</int>   <!-- BRAILLE PATTERN BLANK -->
         <int>0x3000</int>   <!-- IDEOGRAPHIC SPACE -->
         <int>0x3164</int>   <!-- HANGUL FILLER -->
         <int>0xFEFF</int>   <!-- ZERO WIDTH NO-BREAK SPACE -->
         <int>0xFFA0</int>   <!-- HALFWIDTH HANGUL FILLER -->
         <int>0xFFF9</int>   <!-- INTERLINEAR ANNOTATION ANCHOR -->
         <int>0xFFFA</int>   <!-- INTERLINEAR ANNOTATION SEPARATOR -->
         <int>0xFFFB</int>   <!-- INTERLINEAR ANNOTATION TERMINATOR -->
      </blank>
<!--
  Rescan configuration every 30 seconds when FcFontSetList is called
 -->
      <rescan>
         <int>30</int>
      </rescan>
   </config>

</fontconfig>