i dont know if thats a problem or not, but my version tends to do that too....meaning that some vehicles/bikes get quieter when they peak in speed. There are two things I can think of,...try setting your in-game sfx volume down a little. Also, try (if you have XBox, i dunno if this works on PS2) making sure that your selection of sound is supported on your TV (i.e. dolby surround, mono, stereo, etc.). Get back to me on that.