The default music device on the ZK81 is the Zon X-81. It was an add-on that came with an AY chhip and built-in speaker. Many emulators and ZXpand emulate the Zon X-81.
To create music, Andy Rea has created ZXbeta, a music tracker program for the ZX81. It can be used to create your own music or edit some existing music. I have tinkered with it a while back and wrote a tutorial and user guide that I hope helps other use the program.
Once music in created, there three music playera available, JUK and STCPLAYER by Andy Rea and VIP-PLAYER by Siggi. The first two programs play music in an ST file format, while the third plays music in the PT3 file format.
I have discussed them in more depth in ZXzine, but I wanted to mention them here as they are great programs that make the ZX81 still useful.
There is a collection of STC files called BIGSTC that was put together by Moggy.
Both Siggi and Moggy are user names from the SinclairZXworld.com forums, Search for forums for details on all of these programs.