Such a fabulous place; a baroque palace to enjoy a concert in! Going inside you get to take the…read morebeautiful Angel Staircase up to the famous Marble Hall, which is where the concert takes place. I saw a wonderful Mozart concert here with strings and a visiting French Horn player who was fantastic. It's not a huge room and it's nicely intimate inside; the interior is clad with white marble and is gorgeous. I went in March and was able to get a ticket on the same day but I think in summer it must sell out.
They have a coat check and a bar that you can visit before the concert and during intermission. Don't come too hungry though, because I only saw MozartKugel on sale for snacks the night that I attended a concert.
Check on their website a couple days in advance if you can. It's really worth seeing a concert here plus you get to visit the famous Angel Staircase and Marble Hall, which are must sees if you're visiting Salzburg.