[rnranimal]

This is an old thread, but I can explain what happened. I will bet anything the speed correction made was in order to slow down the audio. When you slow the audio down, you reduce the pitch and thus the frequencies get lowered. In digital audio, you have a defined cutoff point. CD would be 22.05khz. So by whatever percentage you reduce the pitch/speed, you also reduce the frequencies. That leaves a gap up top, which would look like MPEG to testing software.

If you speed up the audio, you would be raising the frequencies and unless you raise the sampling rate, you would be loosing some upper frequencies as they will be above the upper cutoff. That's why instead of using speed/pitch processes in audio software, I adjust sampling rate tags without altering the audio to make speed corrections. Just as an example- If you take a 44.1 recording and it needs a 4% increase, you'd change the samplerate tags without altering the audio (Audacity can do this) to 45864khz. All this does is change the rate the audio is being played back and increases the samplerate (or decreases if slowing down). At this point, you have a 45864khz file. You can either resample back to 44.1 and loose some of your gained upper frequencies or upsample to 48khz and retained them (but now have a gap which might look like MP3).