It can be either audio or transcription. It would be perfect if it also contains aircraft tracks.
LiveATC.net has live and recorded ATC radio for most countries/airports. There are lots of flight tracking websites, e.g. FlightAware, that have live and recorded radar/ADS-B tracks. You'll have to correlate the two yourself.
I'm a PPL student and I use two sources:
Yes: Many publicly available accident/incident reports contain communication transcripts; some of those will contain aircraft tracks, too.