Beep (YC S14) did this and made a gorgeous, inexpensive volume dial that lives on top of your speakers and speaks wifi. Unfortunately the company shut down but it was great while it lasted.

It looks like there isn’t market demand for this without the associated speaker business or intelligent assistant.

This one is kind of funny because the speed of sound makes sparse speakers tricky if you’re moving around. It can sound weird.

If you’ve ever seen (and some milliseconds later heard) someone dribbling a basketball at the other end of a long quiet street you’ll know what I mean.

I opted for a low-power FM transmitter fed by the music player daemon (mpd). Zero latency, always synchronized, works with any stereo receiver I want, plenty of clients to choose from (web-based, mobile, etc.) to control the mpd.