voice feedback volume is too low

I've enabled voice feedback in the workout app so that Siri announces my split time and pace. However, the volume is so low I can barely hear it.

I checked Settings > Sounds & Haptics > Alert Volume. It's already turned up to the max. See screenshot:

Does anyone have advice? I've seen a few similar questions in this forum, but in each case the person was directed to the adjust the above setting. Obviously that won't help in my case as it's already maxed out.

For additional context, I have an Apple Watch Series 7 (OS 9) and I'm listening via AirPods (2nd Gen), not the watch's native speaker. I have no other audio apps open or audio content playing while I'm working out. It's just silence that's broken when Siri whispers my split stats. When I do listen to music or a podcast the volume is fine, so this problem appears to be specific to Siri alerts. I can make out the alerts if I'm somewhere particularly quiet, such as a trail, but if I'm on a busy street the road noise drowns them out.