This is well worth the climb - go to the service at 7pm and hear the nuns signing simply beautiful. We thought we'd be the only people at the service, but it was packed and people were standing. It well worth attending this 15 minute service.
One of the most pleasurable experiences I had in Salzburg was listening to the incredible music being performed at the very impressive Cathedral. I happened to be there on the Corpus Christi holiday, and the previous evening there was a Vespers service being celebrated. In addition to the magnificent pipe organ, there was a wonderful choir and orchestra performing a program by Mozart. The experience of listening to Mozart performed in the cathedral where he was baptized and performed himself was memorable and moving. The acoustics are amazing, and because Salzburg is a city with so many talented musicians, the cathedral is a wonderul place to hear excellent music - for free!
The other churches in the city also offer musical experiences; in St. Peter's I heard a reheasal for a choral program that was excellent, You don't have to be a church member or even religious to enjoy these opportunities, but it is good manners to be quietly respectful of those who are there to worship.