Can't help too much on the speaker, I'm afraid. If it were dodgy wiring to the speaker I would expect you to hear crackling, so it might be a dry joint on the board in the radio. But that's as far as my knowledge of electronics goes.
I too had the missing buckle stops. You can buy just the buttons, but although melting the rivet end worked for a while, it wasn't as neat and they came off again. Bought some with a crimping tool on eBay a few years ago, came to about £20. Been fine ever since, so it was an expensive fix for me, but I expect I'll use it again on someone else's car.
John