Definitely G.729/G.729A/G.729B codec won’t cause gaps when playing music, you may check :
Disable Silence Compression – because the CODEC chip maybe cut some time if enabled the silence compression and some part of the music is in low volume ( You can’t feel it but the codec hardware can );
Or Large the music volume – the reason is same to above
Adjust Frames/RTP Package :
What is the Frame number in each G.729 RTP package? I suggest to change to 1 and try again; it use more bandwidth but will make the playing more smoothly;
Welcome post your resule back;