Open the Settings app on your Apple Watch
Tap Sounds & Haptics
Drag the Ringer and Alerts volume slider to the right
Turn the Digital Crown to adjust volume if the slider is selected
Make sure Silent Mode is turned off
If using headphones, adjust the volume from the watch or the connected device
If audio is playing, open Now Playing and increase the volume there
Check that Do Not Disturb or Focus mode is not affecting sound alerts