i saw someone mentioned google service here - you don't need google anything to run OrganicMaps (OM) and get TTS working
also, i didn't thoroughly research this, but there was some concerns with OM at the company/management level, which is why CoMaps (community maps) was forked from OM
this worked for me...
- as someone else mentioned, install SherpaTTS (proTip: don't mess with the volume in the Sharpa app, else distortion)
- accessibility > text-to-speech output > preferred engine - select 'SherpaTTS' even if it's already selected
- comaps > settings > voice instructions - select 'test voice instructions' - if you don't hear the test, then i can't help