First off, if you want to keep the unmodified originals, remember to work only with COPIES. Either that, or burn the originals to DVDs (or memory sticks, or memory cards, or … )
For the MP3s, as I already said, look no further than the excellent freeware program MP3gain. After install, click Options, Advanced, and tick the Enable “Maximizing” features box before clicking OK. You can then load up a big bunch of MP3s and use the Ctrl+M, Ctrl+X ‘trick’ to force it to perform a WAV-style peak normalize to 100% (which it confusingly refers to as 89.0dB—blame the movie industry for that weird standard, apparently >shrug<) on all your MP3s. You will likely find that most of them are ‘over-peak’ by anything up to about 9dB (!).
I use MP3Gain myself for this, and have done for some years now. I rate it as one of my three must-have MP3 utilities (the others being MP3DirectCut, which is freeware; and MP3-Tag Studio, which is cheap but not free.)
For the WAVs, there are several programs which will batch-normalize them. Most are cheapware (up to about $25), but read on.
If you are totally strapped for cash, one slightly desperate option would be to use Audacity. Basically, you open the first WAVfile, Select All, then Effects/Normalize, THEN import another bunch of WAVs and Output them to separate files. (Allegedly. I just read this on the cdfreaks Forum! And personally, I loathe Audacity.)
The only true freeware solution which I’ve seen which can batch-normalize WAVs comes from those well-known Aussies, NCH. I know that many people loathe their DLLs, but in our case all we’ll be wanting is to convert from WAV to WAV but adding a Normalize whilst doing so (if that’s not too bewildering a concept?). They have a freeware program called Switch which claims to be able to convert up to 32,000 files per batch. (No, I haven’t personally tried this program!) You can read all about it at http://www.nch.com.au/switch/ and decide for yourself; I’d say it would fit your particular bill. Look at their screenshots and all will be clear. Provided you do a System Restore first, or use some utility like Returnil, you can always give Switch a whirl with a few files, see if it’s right for you, and have the option of dumping it afterwards if it isn’t.
Hope that helps?