What is the best way to occasionally play a sound?

Simply create a launch agent which repeatedly says something (or plays a sound) and load it:

1. Create a plist with nano in Terminal:

   nano ~/Library/LaunchAgent/usr.home.bose.wakeup.plist
   

2. Copy the following lines and paste it into the Terminal window:

   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
   <plist version="1.0">
   <dict>
       <key>Disabled</key>
       <false/>
       <key>Label</key>
       <string>usr.home.bose.wakeup</string>
       <key>ProgramArguments</key>
       <array>
           <string>/usr/bin/say</string>
           <string>wake</string>
           <string>up</string>
           <string>you</string>
           <string>lazy</string>
           <string>Bose</string>
           <string>SoundLink</string>
           <string>Mini</string>
       </array>
       <key>RunAtLoad</key>
       <true/>
       <key>StartInterval</key>
       <integer>887</integer>
   </dict>
   </plist>
   

3. Save the file with ctrlO and exit nano with ctrlX

4. Load the plist with:

   launchctl load ~/Library/LaunchAgent/usr.home.bose.wakeup.plist
   

You can use other voices by adding the option -v $VOICE. To get the list of all available voices enter say -v ? in Terminal.

Example:

...
<array>
    <string>/usr/bin/say</string>
    <string>-v</string>
    <string>Agnes</string>
    <string>wake</string>
...

The downside of say: you can't set a sound level!

Therefore an alternative launch agent with afplay instead (you can set the sound level with the -v option):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Disabled</key>
    <false/>
    <key>Label</key>
    <string>usr.home.bose.wakeup</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/bin/afplay</string>
        <string>-v</string>
        <string>0.05</string>
        <string>/System/Library/Sounds/Submarine.aiff</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>StartInterval</key>
    <integer>887</integer>
</dict>
</plist>

For this specific speaker, it turns out there is an easier answer: just press and hold the bluetooth and + buttons until it verifies that auto-off has been disabled. Still, thank you all for the input!