Dumb question maybe but why not bring this to other pebbles? There's a mic and you have a network connection via smartphone.
Anyway, even if it didn't, the watch could still just display a speaker/microphone image and a graphic indicating audio playback.
Honestly, this is my personal hold-up on buying a smart watch. I'm pretty set on getting one with Alexa built-in.
That said, it would be a great partnership for Amazon to connect with the Pebble app on people's smartphones to allow for this. As another poster has suggested, Amazon could even buy Pebble at some point. While as a Pebble owner I'd be sad to see them lose their independence, some solid financial backing wouldn't be a bad thing.