Question

here is a solution to pad a track while mixing with another track.

Sox : merge two audio files with a pad

$ sox short.ogg -p pad 6 0 | sox - -m long.ogg output.ogg

How to use this command mixing multiple tracks?

For example

audio1.wav (no pad)
audio2.wav (2s pad)
audio3.wav (2s pad)
audio4.wav (3s pad)

Answer 1

pad 6 0 prepends 6 and appends 0 seconds of silence, so I assume you only want to prepend silence.

You can do this with a while loop, for example in bash:

cat <<EOF > infile
audio1.wav 0
audio2.wav 2
audio3.wav 2
audio4.wav 4
EOF

while read fname len; do
  sox $fname -p pad $len 0 | sox -m -p long.wav output.wav
done < infile

I would suggest keeping the files uncompressed until your done processing.

Be careful of clipping when mixing, one way to avoid it is to apply -6dB gain to both signals and normalize after, e.g.:

  sox $fname -p pad $len 0 gain -6 | sox -m -p "| sox long.wav -p gain -6" output.wav gain -n