The format for ringtones that can be used with the DE310/DE410 phones is wav using ulaw, 8-bit, 8 kHz, mono.
To convert a standard WAV file to this certain specifications, opensource tools like SoX (Sound Exchange) can be used. This tool is available for Windows, Mac OS X and Linux. SoX is a shell based tool. Therefore you have to use the following command line to start converting your soundfile.
sox inputfile.wav -e mu-law -c 1 -r 8000 -b 8 outputfile.wav
Please be aware that the maximum size of your outputfile is smaller than 40 kb.