Program an ATtiny13 as an audio oscillator with variable frequency and pulse-width
Newbie's Guide to AVR Interrupts
Newbie's Guide to AVR Timers
Part Nine - Pulse Width Modulation (PWM)


These should get you pretty far and the rest you can do with the datasheet. Start building piece by piece, from blinky to waveform to waveform that changes over time to tones. Some source might help with filtering and driving audio outputs (active LPF might do both neatly).

I suggest coming back with more specific questions.