asterisk users - Aug 2010 - MP3Player audio format

Hi,

I've successfully installed Asterisk and placed test calls using the 
MP3Player application.

However I notice that my call quality varies drastically depending on 
which MP3 I use.

Since I'm not changing any settings I'm assuming that the encoding of 
the file makes a difference.

I've tried with both the g729 and alaw codec and they both give the same 
replicable results.

I read that the underlying program mpg123 prefers mp3 files without ID3 
tags.

What is the recommended audio format (bit/sec) for using the MP3Player 
application or does this not make a difference?

Thanks,
  Andy