I'm not sure if they are hard to implement, here you are a couple of suggestions to improve the jammr session window.
1) Besides the “mute” checkbox for every person in the session, you could add a “solo” checkbox, like in many multitrack sound apps; when it's clicked it would mute all other sources except this one. This could be useful to quickly understand who is playing what, to detect noise sources etc.
2) It would be cool to have a text field besides each person's username, this could contain some short info, e.g. instrument being played, mood, a comment about the session, funny jokes etc (the kind of stuff you enter in “status message” of messaging applications).