Mono Audio and audio balance on Apple Watch
If you prefer to hear a combined left+right audio signal out of both audio channels on speakers or headphones connected to your Apple Watch, turn on Mono Audio. You can also adjust the left-right balance of your Apple Watch audio, whether stereo or mono.
Change audio settings
Open the Apple Watch app on your iPhone, tap My Watch, tap Accessibility, then do the following:
Switch from stereo to mono audio: Turn on Mono Audio.
Adjust the audio balance: Drag the slider below Mono Audio.