I know this is tagged 12.04, but it it seems the most relevant question to post this on: I tried both the system settings and Thunderbird settings in this post, with Thunderbird 68.7 (custom install) on Ubuntu 18.04 and sound worked (when you hit the Play button in Edit/Preferences/General/When new messages arrive, for an ogg sound in
/usr/share/sounds/ubuntu/stereo/) - but with extremely low level; even though Ubuntu settings (
gnome-control-center) had Output volume on max:
This got fixed when I started
pavucontrol, and realized Playback/System Sounds was very low, which I then increased:
Only then I realized, that "Alert volume" slider on first screenshot, is linked to the "System Sounds" on second screenshot.
But anyways, now the new message sound from Thunderbird is audible - though still lower than other system sounds, not sure why.
EDIT: in about:config of Thunderbird (Edit/Preferences/Advanced/Config Editor.../I accept the risk!) there are also
media.volume_scale, both by default on 1.0 - I set both of these to 3.0, but it seems to have no effect on the new mail sound ...