PiTiVi v0.13.1.2 running on the 9.04 based version of eeeBuntu on an Asus eeePC 901, PiTiVi also exhibited this behaviour with the previous version.   

Loading short mp4 clips from a Sanyo Xacti camcorder - H264 video and AAC audio, which play fine in VLC.

When I copy the clip onto the time line and play it through video and sound are fine the first time, but if I reset to the beginning and play again the video is fine but the sound cuts out after about half a second and either stays silent to the end of the clip or restarts a few seconds later - this repeats every time I let the clip play through and then reset it, but the duration of the cut out can get less after repeated resets. However, if I rewind to the beginning while the clip is playing, both in the silent section or later in the clip, the sound plays through without cutting out. Again this repeats each time I rewind, until I let the clip play to the end, and stop. Then if I reset to the beginning the sound cuts out again.