IANAL, but...
I beleive using the MC channels for your hold music could be considered rebroadcasting, which would probably be frowned upon, and possibly litigable, but probably not something they would persue, since (I assume) it isn't for profit. There also might be csome variances with that since it would then depend on what channel you used, if, for example, you used a MC channel that played public domain type of music, you might be ok, if you play Top20/new kind of music, then you have the RIAA and its team of soul-crushing lawyers to deal with, and they have sued every thing from 12 yr olds to grandmothers.