Flag 26 is the audio-enable flag. It sounds like it's clear. If you do a SF 26, the beep should work again. It's strange though, as flag 26 should be set when you first power up. Also, I don't think that would affect the test and branch in your original problem.
Is there a command to test everything?
There's no command for that, but there is a module that does that. I have no idea where, or even if, you could get one anymore. Serious users these days are using something like
Diego's Clonix or NoV-64 hardware plug-in modules (I have the Clonix-D) that hold lots of ROM module images in their flash memory, or a
41CL transplant board which has hundreds of modules pre-installed and runs up to 50 times as fast as the original, or the MAXX module which should be available soon (see
https://www.systemyde.com/pdf/maxx-4d_manual.pdf for the manual), or maybe another option or two I might be forgetting. There's also
SwissMicros' DM41x which is a modern replacement that's better in nearly every way but the huge drawback for me is that there's no option for HP-IL.
As you figure out and post more of what does and does not work, hopefully another member here who knows more about the insides than I do will be checking in soon and give you some better guidance than I can.