Thanks for your help. When listening to them on my ipod with background noise I can hardly hear the old files because they are voice recordings.
If you're using mp3 files, there's also a few applications that will let you increase the volume of the mp3 without re-encoding it - e.g MP3Gain
To make WAV files louder, I edit them in Acid Pro 4.... export it with a higher volume, and then repeat (if I need it even louder)