5MHz is more than needed for audio, and there are quite a few radio circuits it could be used for. Sure, it's slow by modern standards, but there's still a lot of work done today that could be done with a 5MHz scope. It's just that a 50MHz scope is more useful, and isn't that much more expensive to make these days, so the economics make it unprofitable to build anything too much slower.
You could certainly learn a lot of basic AC circuit theory at 5MHz and below. You could learn how capacitors and inductors work, how they can be combined into various types of filters, how op-amp circuits can be built using reactive components in the feedback loops, etc.
If it's working, don't tear it down in an irreversible way. At least not until you've got something that works better.