Unable to mix gst-pipeline: and file:// sources in a playlist

Hi,
I am trying to use MediaPlayer and VideoOutput in Qt-QML in order to play .mp3 and .mp4. Due to the platform constraints, I will have to use gst-pipeline for .mp4 extension, and file:// for .mp3 extension. I construct a playlist that contains both types of URLs.
However, once I display .mp4 video, playing .mp3 does not work.
I suspect there is a problem mixing those sources in a playlist. Below is the portion of the code I add url's to the playlist.

MediaPlayer {
      id: audioPlayer

     

playlist: Playlist

{     playbackMode: Playlist.Loop     }

ListView {

   ...

   model: FolderListModel {

   ...

      onFolderChanged: {

         clearPlaylist();

         ...

         if (fileExtension === "mp4")
        

{                  audioPlayer.playlist.addItem("gst-pipeline: filesrc location="+url1.toString().replace("file://","")+" ! qtdemux ! h264parse ! avdec_h264 ! autovideosink");          }

        else
       

{             audioPlayer.playlist.addItem(url1)         }

 

I get following error in the console when I try to play audio

(myapp:1515): GStreamer-CRITICAL **: 18:05:49.551: gst_poll_get_read_gpollfd: assertion 'set != NULL' failed
Error: " filesrc location=/run/media/sda1/YabanciClubPop/KC Rebell feat. Mo� ? BIST DU REAL ? [ official Video 4K ] - Dagi Bee.mp3 ! qtdemux ! h264parse ! avdec_h264 ! autovideosink" : "no element \"Rebell\""

I also get:

Error: "Internal data stream error."

Can I please get assistance about this problem? Thank you.