So this is not supported by evidence from the film, but looking at the script, this is my theory.
In the script, Marty says:
MARTY:Doc Brown's all right — he's just a little hung up on time. A couple of years ago, he showed up at my house and hired me to sweep out this garage of his. He pays me 50 bucks a week, gives me free beer... and gives me total access to his record collection — he's got this great old record collection.
Now, knowing Doc, if he has a great record collection he wouldn't just go out and buy a speaker set, but make his own (which would have to be louder than any other speaker system anyone could ever need, just because it can).
I admit this is a lot of speculation and is drawing from the script rather than the film itself, but it's a reasonable theory based on Doc's character.