FM radio with an ATTiny:
https://spritesmods.com/?art=avrfmtx
Something similar: https://www.youtube.com/watch?v=7t7_naYJnHo Source code can be found here: https://github.com/spookysys/attiny-synth
Something similar: https://www.youtube.com/watch?v=7t7_naYJnHo Source code can be found here: https://github.com/spookysys/attiny-synth