Greetings Maxdy,
You might try this site:
http://audacity.sourceforge.net/windows.php
It's a freeware audio program that allows you to open, edit and save audio files of varying formats including mp3 and wav.
As far as small wave files go, there is one format which might suit your needs: Microsoft adpcm wav format.
ADPCM stands for Adaptive Differential Pulse Code Modulation. ADPCM is a lossy compression mechanism. There are various flavors of ADPCM. This particular algorithm was suggested by Microsoft; its quality is similar to IMA ADPCM. MS ADPCM compresses data recorded at various sampling rates. Sound is encoded as a succession of 4-bit nibbles. Each nibble represents the difference between the current sampled signal value and the previous value. The compression ratio obtained is relatively modest: 16-bit data samples encoded as 4-bit differences result in 4:1 compression format.
Microsoft ADPCM is directly supported on most Windows implementations as a native format. Although the quality of IMA ADPCM voice files is not great, the files are portable. There is a real advantage in having compact files that can be played on most Windows PCs.
Windows supports Microsoft ADPCM ".wav" files at sample rates between 8 and 41KHz.
http://www.xentec.be/products/vox_studio/help/microsoft_adpcm_format.htm
I work with audio all the time on my home system. I use Adobe Audition (formerly Cool Edit Pro which Adobe bought.)
Just for fun, I converted a 16bit stereo 44.1khz wav which was 39.5 MB (approx 4 min song) to a 8bit mono 8 Khz wav. This brought the file size down to about 1.8 MB. Then I saved it as a ADPCM wav which brought the size down to 930kb. The quality is not so good. It sounds like what you would hear from an AM radio without the static and other noise. It's not so bad considering the file is about 42 times smaller.
I haven't tried audacity yet but I just downloaded it to check it out. In the mean time, you might want do a google search for utilities that can save files as ADPCM Wavs. Hope this helps.
Edit: For audacity to load/save mp3s, you also have to download the lame mp3 encoder library. It's on the same page as Audacity.