install eix through either emerge or equo/spritz, run "update-eix", and then run eix tuxguitar. That should spit you back a list of USE flags. Post back here if you see arts or jack on that list (please note not -arts or -jack). If there's nothing on the list to that effect you could always try running it from a command line to see if the debug messages give you any info.
When you determine which sound server is necessary, just start the right one - if it's arts it should launch at startup (but barring that can be launched just by invoking artsd with some arguments), if it's jack then you can actually plug it into arts - see this threadhttp://readlist.com/lists/lists.debian.org/debian-user/7/39753.html