無料オンラインGSRTからOGGへのコンバーター

Grandstream ring-tone files. Whilst this file format can contain A-Law, μ-law, GSM, G.722, G.723, G.726, G.728, or iLBC encoded audio, SoX supports reading and writing only A-Law and μ-law.

This format is free, which apparently is one of the reasons for its popularity. The used codecs supports variable bit rates (as a default), and .ogg files usually play back easily on all platforms. All things being equal, the sound quality exceeds that of mp3 somewhat.