/usr/bin/vboxplay is in isdnvboxclient 1:3.25+dfsg1-3.3ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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##
## vboxplay v2.0.0BETA5 (17-NOV-98)
#----------------------------------------------------------------------------#
# Usage: vboxplay SAMPLENAME [ VOLUME ]                                      #
#        If you want to change this, copy it to /etc/isdn/vboxplay and edit  #
#        the copy there.                                                     #
#----------------------------------------------------------------------------#
if [ "$0" != /etc/isdn/vboxplay -a -x /etc/isdn/vboxplay ]; then
	exec /etc/isdn/vboxplay "$@"
	echo "Can't execute /etc/isdn/vboxplay"
fi
SAMPLE=$1
VOLUME=$2
if [ -z "$VOLUME" ]; then
   VOLUME=5;
fi
   # Replace this with your own play command. You also can use the pvftools
   # (or some other) to convert and play the messages.
# check whether NAS is running, and adjust DOPLAY to correspond
# check two locations, apparently it moves around...
PLAY=/auplay-not-found-so-dont-use-it
[ -x /usr/bin/auplay     ] && PLAY=/usr/bin/auplay
[ -x /usr/bin/X11/auplay ] && PLAY=/usr/bin/X11/auplay
if [ -x $PLAY ]; then
	AU=/au-not-found-so-dont-use-it
	[ -x /usr/bin/au     ] && AU=/usr/bin/au
	[ -x /usr/bin/X11/au ] && AU=/usr/bin/X11/au
	if ps aw | grep -v grep | grep $AU > /dev/null; then
        	DOPLAY="$PLAY -audio localhost:0 -volume $VOLUME"
	else
		PLAY=/auplay-not-usable-so-dont-use-it
	fi
fi
if [ ! -x $PLAY ]; then	# try aplay (from alsautils) instead
	if [ -x /usr/bin/aplay ]; then
		PLAY=/usr/bin/aplay
		# if anyone has a better suggestion,
		# send email to isdnutils@packages.debian.org!
                # thankfully, aplay understands '-' to mean stdin
		DOPLAY="$PLAY -q -"
	fi
fi
if [ ! -x $PLAY ]; then	# use play (from sox) instead
	# we need to convert to decimal for 'play'. Try 'dc'.
	if [ -x /usr/bin/dc ]; then
		soxvolume=`echo "2k $VOLUME 100/ps." | /usr/bin/dc`
	else
		# fall back to (slower) perl, which is always there
		soxvolume=`/usr/bin/perl -e "printf '%.02f', $VOLUME/100"`
	fi
        DOPLAY="/usr/bin/play -t au - -v $soxvolume"
fi
/usr/bin/vboxmode $SAMPLE
case $? in
   2|3|4|6) /usr/bin/vboxtoau <$SAMPLE | $DOPLAY
            ;;
         *) echo "$0: unknown vbox message format!"
            ;;
esac
# Example with pvftools:
#
# vboxtoau <$SAMPLE | \
#           autopvf | \
#           pvfamp $VOLUME | \
#           pvfcut 0.20 | \
#           pvftoau 8000 | \
#           auplay
 |