Linux Mint and sound problems

Sometimes the desired sound output source (or "sink" using the sound setup terms, I think) isn't selected.

Linux Mint 18 should still use pulseaudio as 17 does.

Have you selected that card (or tried everything listed) in the Sound Settings under output? (on xfce it's accessible by clicking on the sound/volume icon in the toolbar indicator plugin, other DE's should be similar)

Or use the terminal and the pactl command could do about the same thing:
pactl list sinks should show your available outputs ("sinks"), and
pactl set-default-sink [name] should set one as default, using the Name: from above.