The Avalanche's implementation of G729 is G729AB which includes VAD (voice activity detection). When decoding a raw file extracted from the pcap, the stream/voice could be distorted if the decoder cannot convert raw file into pcm format because of the VAD feature. This is a decoder problem and not releated to the G729AB codec.
The Avalanche currently does not support G729A without VAD.
G 729 G.729