Something rain resistant would be good.
Theft resistant (or detachable) would be good too.
I've seen some boom box type things that claimed to be rain resistant, but I can't remember where. Most of them can run on 12v from a dc-dc converter.
That's what I have in mind. Something that can go into the BATTERY BOX and be waterproof plus secured along with the LiFePO4 Battery pack which would also be considered a "high theft" item.
I don't know what will work but the little radio Sony makes with a speedometer computer won't do well unless you are in a metro area and don't exceed 18 MPH or so. After that the wind noise drowns it out. Go slow and it's fine, fast and turn it off cause you can't hear it anyway.
Mike
You could put the amp and the speaker behind and/or under the seat then just plug an ipod or other mp3 player into it with a cord going to a pocket or some custom holder by your hip maybe.
Can imagine that on a trailer behind the ebike but no power to move you @ all
But as for sound why not just an ipod or other mp3 player with head phones at a reasonable volume to hear other traffic
Although I use to have a cheap transistor radio hooked up to a dynamo on the front wheel took it of the bike when not using it (about 20 years ago thou)