If you happen to be flipping through radio channels tonight, you may hear something pretty unexpected in between all of the usual pop suspects: a harpsichord. No, it's not the product of some new über-hipster band. The harpsichord is actually just one part of a small orchestra performing "The Ethiopian Ball," a new 70-minute radio "comic opera" created by father-son team Eric and Roger Beaumont and premiering on WMSE tonight at 6 p.m.