Hi yyz,
There are two buffers involved, in input buffer that is very small (reading the file in) and an output buffer (data being sent to the sound card). Data is read in, and the entire song is usually read in long before the file is done playing. However unless you only have one song in your playlist, the BDP will begin to read in the next song while the first is playing. You shouldn't have any issue playing back audio files, if you do it should be easy to troubleshoot if its the network or not as there is a benchmarking utility built in to inidicate available bandwidth between the bdp and the NAS.
Cheers,
Chris