One of the programs included in the Nokia PC Suite is the Multimedia Player. (If you don’t have one, you can download an installer over the net. Here’s the direct dload address: http://nds1.nokia.com/phones/files/software/Nokia_6600_Multimediaplayer_1_1_en.exe. The program is freeware. Right click and “save target as…” to download)
To convert, simply open your target file (wav or MP3), and then go to File>Save as… Select either AMR or MP3, and then select your size limit (MMS size, 200KB, 300KB or none). Click save and that’s it.