I have some microphones I am planning to sell and I would like to make some recordings with these microphones so that the buyer will have a good idea what they are getting.  I am thinking of a simple setup where I use a studio monitor to play a reference file.  I will record all the mics using exactly the same setting (same distance from the speaker etc), and also do the same with a well known microphone such as a Beta 58.
I realize this isn't the kind of test an acoustician might do, but I think it will serve my purposes as a good faith demonstration of how the microphones work.
My question is what to use as a reference sound file.  I expected I could find such a thing with a Google search, but I have come up empty.  I could use commercially recorded songs that have a range of dynamics and wide frequency spectrum.  But I was thinking more like a test pattern.
Any ideas?

This is not a smart way to sell a microphone.  Too many variables. The speakers, the room, the reference material itself, distance from speaker to mic.  I personally would not be impressed with this sort of assessment.
 
I would be recording things with the mics I am wanting to sell. Try and pick things that suit the microphones really well. Vocal mics, acoustic guitar, drums etc.. Are there any acoustic things you could record.  That is often a good way to tell.
 
I would be keeping microphones as well unless you really need the money.  They will always come in handy.  Especially on a bigger sessions where you suddenly need every microphone you have to handle it.

Jeff Evans
This is not a smart way to sell a microphone.  Too many variables. The speakers, the room, the reference material itself, distance from speaker to mic.  I personally would not be impressed with this sort of assessment.
 
I would be recording things with the mics I am wanting to sell. Try and pick things that suit the microphones really well. Vocal mics, acoustic guitar, drums etc.. Are there any acoustic things you could record.  That is often a good way to tell.
 
I would be keeping microphones as well unless you really need the money.  They will always come in handy.  Especially on a bigger sessions where you suddenly need every microphone you have to handle it.

I guess I should have mentioned this is a low-end VocoPro 5800 wireless microphone kit. I am not trying to make it sound better than it is.  I am trying to do a full disclosure so that the buyer will not be disappointed.
 
I ended up using SONAR to string together some short samples of different genres that showed the extremes of frequencies and dynamics.  It was instructive to me.  While all 4 VocoPro mics sound very similar -- practically identical, the first one is about 2dB hotter than the other 3 with all of them at the max setting on the VocoPro receiver.  That's not a big deal.
 
I compared against an SM58 wired.  The Shure sounds better, of course, but not much better in most of the material.  I can really tell a difference above about 9K -- the VocoPro is not nearly as crisp.  And on a loud very high trumpet passage, the VocoPro had a really strange "corona" around the sound that was not there on the Shure.  I attribute that to the cheap wireless design of the VocoPro.  The VocoPro is weaker below 80 Hz also.
 
The VocoPro has a very poor reputation among serious sound engineers.  But judging strictly from the sound quality, it isn't nearly as bad as its reputation.  But there are other issues.  It is extremely sensitive to feedback.  It is just fine for rappers who eat the mic, but probably not worth the trouble for anybody else. 

Ok well that is quite different. Not sure how you would go about it other than set it up through the wireless system and perhaps record some vocals/voice and state the recording is over the wireless connection. Maybe just beef up the recording a little with some EQ to make it sound as good as it can.