Very muffled audio

I have a Plextor drive and I use the bundled Nero Express to burn CD-Rs. I have been doing this with no problem for a while

However I am now getting some CD’s which give a very low-level muffled and bassy audio.

This is what I do.

Download MP3 files from whatever source (I use several different ones)
Rename them in the seqence I want them to be in on the CD
Open Nero and select Audio CD
Drag and Drop all MP3s in a single batch to the open Nero window (it says its analysing each file, looks like its converting them to WAV’s on the fly)
Burn the CD.
Burn completes without errors.

The finished CD plays OK in three different computers using CD Player with good sound, but in my stereo, it plays with very low level muffled audio. I have even tried playing it on my DVD player and piping it through the Aux Input of the stereo, which normally works OK for other CD’s I have burned. Same result; very low level muffled audio.

Any suggestions guys?

Burn slower.