There can be a number of reasons. Frequently it's caused by compressing the audio at the encode.
Were these files created on the IPTV server, or imported from another capture/production system?
Are they MPEG1 or 2 (or 4, or H.XXX whatever).
Some details would be helpful, but, the short version is: It's not the network, it's the production of the video.
FWIW
Scott