Used to mock someone's voice as being strange, unpleasant, or inadequate (e.g., for singing). Bacon, being a dietary staple in older times, was often used as a metaphor for financial stability or wealth; having the voice of one who must "beg bacon," then, means having a harsh voice, like someone who is undernourished. Did you hear the way that singer was screeching last night? I'm glad we didn't stay too long, he had a good voice to beg bacon.