I found that all asterisk sounds have a gsm extension. Me and my VOIP provider are using alaw codec. Does it mean that I have to recode all sounds I’m using in alaw format?
no. asterisk will send alaw data regardless of what the extension format is. The only time you’d want to convert prompts is if you use a high-CPU codec (G.729, iLBC, etc) for a voicemail/conference box. Encoding the prompts into the target format saves the CPU power of encoding them constantly on the fly.