Because telephone lines are limited to 8 khz, all sounds that you import will be down-converted to an 8 khz format.
If you have some wave/audio editing software, you may consider recording or down-converting the file to 8 khz mono pcm before loading it into IVM and this shouldn't require IVM to make any changes to the way it sounds.
But yes, 8khz is pretty universal for telephone lines and it will sound much worse in comparison to your original 22 or 44 khz recording, but at least it gives you an idea of how the audio will actually sound on the phone.
The best thing to do is to record the prompts in 8khz format from the beginning, you'll find it will retain more quality. It's the same reason that many text to speech engines come in both a 22 khz (for normal speech) and an 8khz format (for phone prompts).