Miksoft Mobile AMR Converter is PC application that can convert wave music (*.wav) to amr (*.amr) and vice versa. I'm just using it to convert wave to amr, because if converted wav from convertion amr to wav, the quality isn't good. This application was made by Miksoft. Actually, as you see the picture above, this application was made for Sony Ericsson phones, but since all S60 phones and some S40 phones had support amr file, you can use this.
Well, I use amr file for my Nokia 3200, since it can't play mp3 files, but you could use this also for the reason of space, because amr file has small size than mp3 file (there was a phone that support mp3 files but didn't have memory card). To use this application to convert a wav file to amr, you should have wav file that has property 16 bit with 8 kHz sampling frequency to convert (to have this, you could you music converter application like I used), because if you use another wav setting, you'll get a worse quality amr file.
Some phones, like my Nokia 3200, has limit for a file size, my Nokia 3200 has limit about 50 kb per files. So, if that's your case (like mine), you should cut the music for only some second that has a part you like and convert it (to do this you can use music editor like I used).