Performing music together over an Internet connection is next to impossible if there is *any* lag at all. Typically, the lag is caused by short 10-50 millisecond delays for every router hop, at minimum one hop from you to your ISP, another from ISP to Internet Exchange Point (IXP), IXP to my ISP, and a fourth from my ISP to my computer. When you play the drum I hear your drumbeat 40-200 milliseconds after you play it. When I play a note on the piano you'll hear it 40-200 ms after I play it, or 80-400 ms after you've played your drumbeat. And our friend @StrangeAttractor will hear your drumbeat and my piano out-of-sync by 40-200 ms; if she plays her guitar synced to my piano you'll hear that 120-600 ms after you've played your drumbeat.

There was a really cool commercial by Bell some 10 years ago that showed people playing together over their Internet connection; sadly it was only movie magic and no more realistic than flying through the air with a cape.