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