I wouldn't get my hopes up on Public Domain songs. The Public Domain is sadly not as clear-cut as one might thing and plenty of songs that seem public domain aren't, as MANY artists have learned in the past. And there is the well-known example of "Happy Birthday" which is NOT in the public domain though certainly seeming like it must be. I would imagine it being likely that whatever requirements HMX puts in place to prohibit cover songs would really extend to any song where publishing rights don't emanate from the artists, including "traditional" songs. This is really a better safe than sorry issue and with the difficulty in verifying that a song is public domain and the additional issue of copyrights on arrangements of songs in the public domain, HMX would be reasonable if they wanted to steer clear.