The scaler in the Pioneer plasmas, and probably in other brands as well, causes the audio to be ahead. The video processing uses up that ~150ms and causes the audio to slip ahead.
Are you using HDMI from the 3800 into the 5308, or are you using analog interconnects? You might experiment with getting audio from both of these options and see if it helps. Otherwise, it's really not a problem if you adjust the audio delay in the 3800 to sync the sound and picture. Unless the 5308 allows you to set the audio delay for each input independently, you won't want to adjust the delay there. That will cause ALL your audio/video sources to have that delay (satellite TV, etc.) Now, if ALL your AV sources exhibit the sync problem, then you can adjust it in the 5308.
Hope this helps.