diff --git a/bin/muxsa-pngaac2mp4 b/bin/muxsa-pngaac2mp4
index e4cb2cc..587eed7 100755
--- a/bin/muxsa-pngaac2mp4
+++ b/bin/muxsa-pngaac2mp4
@@ -38,41 +38,94 @@
 
 echo "This is muxsa-pngaac2mp4." 
 
-OUTFILE="out.mp4"
-if [ "$1" ] ; then
-    OUTFILE="$1"
-    # make sure that file ends on .mp4
-    OUTFILE="${OUTFILE%%.mp4}.mp4"
+# first, we need to define 3 file names - check command line, then defaults:
+# SLIDES_IN - concat multiplexer control file with png slide names
+# AUDIO_IN  - one .wav or .m4a sound file, or concat file with several wav/m4a
+# VIDEO_OUT - mp4 output
+
+while getopts "a:s:o:" OPTION ; do
+    case "$OPTION" in
+    a)
+        if [ -r "$OPTARG" ] ; then
+            AUDIO_IN="$OPTARG"
+        else
+            echo "error: AUDIO_IN file $OPTARG does not exist. abort."
+            exit 1
+        fi
+        ;;
+    s)
+        if [ -r "$OPTARG" ] ; then
+            SLIDES_IN="$OPTARG"
+        else
+            echo "error: SLIDES_IN file $OPTARG does not exist. abort."
+            exit 1
+        fi
+        ;;
+    o)
+        VIDEO_OUT="$OPTARG"
+        ;;
+    *)
+        echo "Incorrect options provided - ignored"
+        ;;
+    esac
+done
+shift "$((OPTIND-1))"
+# backward compatibility: before getopts we only accepted OUT as parameter
+if [ -z "$VIDEO_OUT" -a -n "$1" ] ; then
+    VIDEO_OUT="$1"
 fi
 
-if [ ! -f slides.concat ] ; then
-    echo "error: file slides.concat missing. abort."
-    echo "consider using muxsa-al2fc for creating one."
+if [ -z "$AUDIO_IN" -a -r "soundtrack.m4a" ] ; then
+    echo "Setting AUDIO_IN to default file name soundtrack.m4a"
+    AUDIO_IN="soundtrack.m4a"
+fi
+if [ -z "$AUDIO_IN" -a -r "soundtrack.wav" ] ; then
+    echo "Setting AUDIO_IN to default file name soundtrack.wav"
+    AUDIO_IN="soundtrack.wav"
+fi
+if [ -z "$AUDIO_IN" -a -r "audio.concat" ] ; then
+    echo "Setting AUDIO_IN to default file name audio.concat"
+    AUDIO_IN="audio.concat"
+fi
+if [ -z "$AUDIO_IN" ] ; then
+    echo "No input audio file given and no default file name found. abort."
     exit 1
 fi
 
-if [ -f soundtrack.m4a ] ; then
-    AUDIOFILE="-i soundtrack.m4a"
-    AUDIOOPTS="-c:a copy"
-else
-    if [ -f soundtrack.wav ] ; then
-        AUDIOFILE="-i soundtrack.wav"
-        AUDIOOPTS="-c:a aac -b:a 128k"
-    else
-        if [ -f audio.concat ] ; then
-            AUDIOFILE="-f concat -i audio.concat"
-            AUDIOOPTS="-c:a aac -b:a 128k"
-        else
-            echo "neither soundtrack.m4a, .wav, nor audio.concat found. abort."
-            exit 1
-        fi
-    fi
+if [ -z "$SLIDES_IN" -a -r "slides.concat" ] ; then
+    echo "Setting SLIDES_IN to default file name slides.concat"
+    SLIDES_IN="slides.concat"
+fi
+if [ -z "$SLIDES_IN" ] ; then
+    echo "No input slides file given and no default file name found. abort."
+    exit 1
 fi
 
-ffmpeg -f concat -i slides.concat ${AUDIOFILE} \
+# now we got all input and output file names
+
+# depending on the type of the audio input, we need to give different options
+case "${AUDIO_IN##*.}" in
+    m4a|M4A|aac|AAC)
+        AUDIO_SOURCE_OPTS="-i"
+        AUDIO_CODER_OPTS="-c:a copy"
+        ;;
+    concat|CONCAT|sequence|SEQUENCE|seq|SEQ)
+        AUDIO_SOURCE_OPTS="-f concat -i"
+        AUDIO_CODER_OPTS="-c:a aac -b:a 128k"
+        ;;
+    *)
+        AUDIO_SOURCE_OPTS="-i"
+        AUDIO_CODER_OPTS="-c:a aac -b:a 128k"
+        ;;
+esac
+
+# time to start ffmpeg
+# no "quotes" around the $..._OPTs on purpose
+set -ex
+ffmpeg -f concat -safe 0 -i "$SLIDES_IN" $AUDIO_SOURCE_OPTS "$AUDIO_IN" \
     -vsync cfr -vf fps=25 \
     -c:v libx264 -preset slow -tune stillimage -crf 23 -pix_fmt yuv420p \
-    ${AUDIOOPTS} \
-    -movflags +faststart -brand mp42 "${OUTFILE}"
+    $AUDIO_CODER_OPTS \
+    -movflags +faststart -brand mp42 "$VIDEO_OUT"
 
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