Not sure if it's rare (someone on eBay literally is selling new copies for $5), but I think it's definitely obscure. The Blaze mp3 CD player for the DC allowed Dreamcast users to play mp3 CDs (which held a lot more songs, since mp3s are more compressed than CD audio is) on the DC. I'm playing it right now as I upload a video, and it's...alright, I guess? The user interface is atrocious, and it won't work with VGA, but it's alright for what it does, I guess.
Only question I ask is... Why? If you want to listen to music, put a disc in a boombox or stereo. Running two devices to listen to a song is silly.
Most boomboxes and stereos of the time couldn't play mp3 CDs either, it was pretty much the DC or a PC for a few years.
There was MP3 players at the time though. So why bother using a console and power two devices.. It just seems pointless.
There was a homebrew mp3 player that displayed songs on the VMU so TV was not needed. I used it quite a bit at the time
It may have displayed the song title and artist on the VMU screen, but where is your audio outputting?
Now that just sounds wildly unrealistic. Quite the risky guess indeed. My question was just going back to the whole multiple devices point.
Jokes aside, I guess it was useful as an mp3 player back when it wasn't that common, hooked to a home theater you could keep the tv turned off and still listen to the songs via the speakers and have the player status via the VMU.
MP3 players in 2001 usually had something like a whopping 32 MB of memory, and they weren't that cheap either.