IS it only a particular brand of disc?
How long have you had the 5006? Dust may have built up on the lens, if so a lens Cleaner may help.
Compatibility between players is a long standing issue, long before the Liteon recorders appeared hence why firmwares are always being upgraded, for instance my old pioneer player was extremely picky about media until I upgraded the fw on the 2 burners in the pc (nec 1300 & 3500) once I did that the pioneer has so far accepted without fault all discs burnt on the pc since.
What firmware are you running in the 5006?
Does the Liteon play any discs without the sound problem?