In case you were not aware, when you're on a call on your Apple Watch, you can adjust the volume by turning the Digital Crown (the round button on the side of the watch). Make sure you're turning it while on the call to increase the volume.
Here are a few things you can try:
- Check the Volume Settings: On your Watch, tap on the Settings app. Scroll down and tap Sounds & Haptics. Use the slider under Alert Volume to increase the overall volume on your watch.
- Check to make sure the speaker on your Apple Watch isn’t blocked or covered. Dust or debris can sometimes reduce sound quality.
- Try restarting the Watch.