If you've been collecting iTunes or MP3 files for any length of time, you've probably gathered some duplicates. Programs that compare file size, file names, and tags are largely effective in rooting out these storage wasting redundancies, but they're not perfect. For instance, you may have renamed a tagless file and re-encoded it to another format without removing the original. Audio Comparer ($30, 30-day free trial) takes the process a step further and as you might've guessed from its moniker--compares the actual audio.
Audio Comparer does this faster than I thought it might, processing about 140 files in approximately two minutes. Alas, support is a tad limited: MP3, MP2, MP1, WMA, AIF, WAV and OGG are a start, but so many people using iTunes, Audio Comparer's lack of support for AAC is regrettable. To test the program's mettle, I re-encoded an MP3 file of Red Ryder's "Lunatic Fringe" to Ogg, as well as twice to MP3 with different names with all the tags removed.
The results were eventually rewarding but initially a bit disappointing. I'm not sure what algorithm is being employed, but I suspect something in the way of a dynamics snapshot. The program rated new wave band The Fixx's 1983 "Sign of Fire" as an 89% match for the rock song "Lunatic Fringe." Both tunes have a lengthy silence and slow volume increase at the front, but little else in common. More bothersome was that the program didn't find the re-encoded versions of Lunatic Fringe on the first or second passes. It found them the only third time I had the program process the folder in which they resided.
Audio Comparer did the job on the third try, so I'll assume it was a glitch and give it the nod. Just make sure you run multiple passes. The 30-day trial/demo doesn't move, copy, or delete files (which is to be expected); however, it does tell you the name and location of the matching files.