/etc/X11/Xsession.d/80_ltsp-sound is in ltsp-server 5.3.7-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 | if [ -x /usr/bin/asoundconf ]; then
    if [ -n "$LTSP_CLIENT" ]; then
            /usr/bin/asoundconf set-pulseaudio
    else
        if [ ! type pulseaudio > /dev/null 2>&1 ]; then
                /usr/bin/asoundconf unset-pulseaudio
        fi
    fi
fi
 |