Let me clear up some bitrate misunderstanding for him
.wav format is called Raw PCM in other words, witch means, its a basic losless coding used by CD-s. So the data is not shrinked.
in .mp3 and other formats, tha data is shrinked, and coded, so files are smaller.
The bitrate shows how big was the shrinking algorithm (so it nearly indicates how much data you can expect to loose in the coding) when the original losless source is shrinked.
MP3 encoder, when codes a track, analises it, and cuts down some of the lower and higher frequencies, so it does not have to store those frequenci ranges (less data).
If you go lower on the bitrate more frequency ranges are cut off.
As far as I know, for exmaple in the 128kbps bitrate, you have nothing over 16kHz, and below 60 or 70 hz.
But the point is: If you convert these tracks to a higher bitrate, you will not get better quality. In fact: Sometimes recoding a track to a higher bitrate also lowers the quality a little bit.
So as you said, just get them in a bigger bitrate, and you should be ok
Seriously if the tracks sound quality is not to your likeing, then the track is useless not matter what the bit rate,have come across some tracks that had high bitrates and still sounded bad /different and my ears told me that i needed a replacement 
it does help to have all or most of them on the same quality level, akin to trying to mix with two different phono cartiges, can be done but the outcome is better if there the same,carried two brand new needles mounted in the styles head and changed them out in pairs when one went bad 
