Translation:I was writing a letter when I heard music.
I just want to say, getting the listen and translate version of this, WOW what a test!
The audio is wrong here. It sounds as if there was some additional word like 'Pisałem na list'.
What would it be if it were: 'I was writing a letter whilst I was listening to music'? ie can you clarify difference in forms between continuous action and a finite action that interrupts it (as this is - is it the 'u' prefix that makes it finite?) and continuous action with contemporaneous continuous action. Thank you!
"usłyszałem" clearly refers to the moment when, suddenly, I heard something. So it's not exactly finite (you can finish listening to some song, some album, but not exactly 'music', that would be for example "Przesłuchałem najnowszą płytę IAMX" = "I have finished listening to the newest IAMX album").
Your sentence is "Pisał[em/am] list, słuchając muzyki" - it uses a participle form.