How to reproduce:

Write a script (e.g. called “test-script.sh”) with this content and set the executable bit:

set -e
set -x

echo "stdout"
echo "stderr" >&2

Then specify that script in an mount point of the icecast configuration like this (I have this in my mount point defaults):

        <on-connect>/path/to/test-script.sh</on-connect>

The beginning of the dump-file will look like this:

# hexdump -C /srv/icecast/test-stream/backup.mp3 | head
00000000  2b 20 65 63 68 6f 20 73  74 64 6f 75 74 0a 2b 20  |+ echo stdout.+ |
00000010  65 63 68 6f 20 73 74 64  65 72 72 0a 73 74 64 65  |echo stderr.stde|
00000020  72 72 0a 2b 20 65 78 69  74 20 30 0a […]          |rr.+ exit 0.[…]|

This makes the dumpfiles difficult to use as backups of the stream data.

I think the <errorlog> target would be more a appropriate target for this output.