When you have an Apple Watch synced to your iPhone, you can use its built-in features and buttons to control music playing from your iPhone. Conveniently, when your AirPods are also synced to your iPhone, they’re considered an extension of the phone itself, so any music you’re playing (and adjustments you make) will automatically reroute through them.
Here’s how to adjust the volume through your Apple Watch.
Open the Music, Podcasts, or Books app on your iPhone and start a song/podcast/audiobook.
On your Apple Watch, open the Now Playing app.
With the Now Playing app open, turn the Digital Crown on the Apple Watch’s side to manually adjust the volume.
Since you have a physical dial to fiddle with, this is a much more precise way to adjust the volume than swiping on the stems of your AirPods.
While the music should reroute automatically through your AirPods, if it doesn’t, your music may be playing on a different speaker. You can fix this through the Apple Watch.
Open the Now Playing app on your Apple Watch.
Tap the left-pointing arrow on the top-left of the screen to bring up the device list.
Find and tap on your iPhone to set it as the active device.
Playing audio through your iPhone should route through any connected speakers, i.e., your AirPods.