TL;DR

Command line magic to the rescue!

ffmpeg -i input0.wav -i input1.wav \
  -filter_complex \
  "[1]    aeval  = -val(0) | -val(1)                     [a]; \
   [0][a] amerge = inputs=2,pan=stereo|c0<c0+c2|c1<c1+c3 [b]" \
  -map "[b]" -c:a pcm_s16le output.wav

Variations:

• for output as an .aiff file, use pcm_s16be instead
• for multichannel audio diffs¹, use aeval = -val(0):c=same

Background

Sometimes I want to diff two audio files when working on mixes, because I forget what has changed between versions. I keep notes, but they can be subtle or hard to verify.

The above FFmpeg command² outputs the differences between two files. It works on both WAV and AIF input files. I haven’t tried too many variations³, but it’s working for stereo 16-bit 44.1k and 48k files just fine.

Note the command outputs the diff in BOTH input files to the output. So if something was nudged over by 1 beat, you’ll hear the original start and the new one start over each other. That is still helpful in drawing attention to what has changed.⁴

Explanation (from Claude Sonnet 4.6)

Inputs:

• -i input0.wav and -i input1.wav — the two input files, indexed as [0] and [1]

Filter chain:

• [1] — applies the following filter to input 1
• aeval — evaluates a math expression on each audio sample
• -val(0) — negates the value of the left channel
• -val(1) — negates the value of the right channel
• | — separates the expressions for each channel
• [a] — labels the phase-inverted output stream as a
• [0][a] — feeds input 0 and the inverted stream into the next filter
• amerge=inputs=2 — merges the two stereo streams into a single 4-channel stream
• pan=stereo — remixes that 4-channel stream back down to stereo
• c0<c0+c2 — new left channel = original left + inverted left
• c1<c1+c3 — new right channel = original right + inverted right
• [b] — labels the final output stream as b

Output:

• -map "[b]" — tells FFmpeg to use stream b as the output
• -c:a pcm_s16le — encodes as uncompressed 16-bit PCM, little-endian audio (for WAV)
• output.wav — the output file

Multichannel variation:

• val(0) — refers to whatever the current channel is rather than specifically the left channel and
• c=same — tells FFmpeg to preserve the input’s channel layout in the output, rather than having to explicitly enumerate each channel with |

References