Yes, it's known. Quite a few people have reported bad performance with JACK. I'm not quite sure on the CPU hog though, it hasn't appeared to me that LMMS is using much CPU unless I'm playing back multiple VSTis. The JACK issue should be in the works now, but the 0.3 series is in freeze, there will be no further releases according to one of the developers. Priority is now given to the 0.4 branch, and you can check out the svn build to see how the JACK state is, and then report to either the LMMS-user list or the IRC channel ##lmms on freenode. For Singerbot, you need to have festival (of course, including headers; devel packages) installed. However, it does not work well (or at all) with lvsl (LMMS VST support layer). The app will crash whenever you drag/insert the Singerbot plug-in to a track. I'm not positive this has been fixed in 0.3.2, but in any case you should install the latest. As for the licenses of samples, they're obviously CC (or even GPL) - if not LMMS wouldn't be able to distribute them ;)