Comment 10 for bug 1398560

If alarms and ringtones are playing fine, it means the core part of the audio stack is fine, might be an issue with the volume side of things (e.g. pulseaudio).

When you reproduce the issue, please run the following and paste the output: 'pactl list sink-inputs'

This will list all the active inputs and the current volume for them.

Another thing to try:
1 - Save your ~/.config/pulse (in case it's broken)
2 - Erase ~/.config/pulse
3 - Reboot and retry