I know, it’s not a real synthesizer at all, but it’s more than the standard beeper.
10: POKE 56000, 0, 50, 78, 50, 18, 95, 97, 16, 223, 96, 239, 223, 107, 2, 57, 13, 65, 41, 16, 55
20: POKE 56003,X
30: POKE 56001,Y
40: CALL 56000
It’s a little bit more complicated: line 10 creates a little assembler program, that will be called in line 40. Use X for the tone to be played, Y for the delay.