If you're not actively planning content for your on hold program, and, say, play the radio on hold, you're not controlling what you're playing on hold. You're at the whim of a Radio DJ or program manager, who might play music that's unappealing to your callers or who might be producing and playing ads for your competition.
Humor is a gift you can give to your callers. It lightens the mood, relieves stress, and can even ignite those random explosions of joy known as laughter.

Version 3 NEC DSX systems have the ability to upload a .Wav file directly into the message-on-hold mailbox using the PC Administrator program.
For those still using analogue or digital phone systems, voice on ISDN circuits has a bandwidth of around 4000Hz. The audio from an analogue phone is sampled at 8000Hz, and because of the nyquist theorem, must be low-pass filtered to 4000Hz. So any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are good at recognising human speech, so this is considered "toll-quality" voice, i.e. good enough to pay for. This is the reason why certain vocalised letters are difficult or impossible to tell apart on the phone. For example, the differences between B, P, T, C, etc. are almost entirely above 4000Hz.