Hi
The following steps may help:
On your iPhone, in the Watch app, go to: My Watch > Sounds & Haptics:
- Check that the Alert Volume is towards the centre or right hand side.
- Check that Silent Mode is not enabled.
Check that Theatre Mode is not enabled on your Apple Watch:
Remove any case, bumper or cover that you may be using with your Apple Watch.
Clean your watch, paying particular attention to the speaker, in case it has become blocked by debris.
If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:
Restart both your iPhone and your Apple Watch, turning both off together, then restarting your iPhone first:
If the issue persists, next try unpairing and re-pairing your watch:
If the issue then still persists, I suggest that you contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:
Apple Watch Service Answer Center - Apple Support