ERROR: from element /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0: Your GStreamer installation is missing a plug-in. Additional debug info: ../../../gst-plugins-base-1.12.2/gst/playback/gsturidecodebin.c(998): no_more_pads_full (): /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0: no suitable plugins found: ../../../gst-plugins-base-1.12.2/gst/playback/gstdecodebin2.c(4600): gst_decode_bin_expose (): /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0: no suitable plugins found: Missing decoder: MPEG-1 Layer 3 (MP3) (audio/mpeg, mpegversion=(int)1, mpegaudioversion=(int)1, layer=(int)3, rate=(int)44100, channels=(int)2, parsed=(boolean)true)ERROR: pipeline doesn't want to preroll.  

raspberrypi3:/media/Transcend# 4a-play 01-Human.mp3 
Play '/media/Transcend/01-Human.mp3' on 'hw:Loopback,0,2'
The selected card (hw:Loopback) is handle by 4a, call open on 'multimedia'
Detected systemd unit file!
Port detected: 31002
ON-REPLY 1:ahl-4a/multimedia: ERROR
{
  "jtype":"afb-reply",
  "request":{
    "status":"Already opened!"
  }
}
ERROR: from element /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0: Your GStreamer installation is missing a plug-in.
Additional debug info:
../../../gst-plugins-base-1.12.2/gst/playback/gsturidecodebin.c(998): no_more_pads_full (): /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0:
no suitable plugins found:
../../../gst-plugins-base-1.12.2/gst/playback/gstdecodebin2.c(4600): gst_decode_bin_expose (): /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0:
no suitable plugins found:
Missing decoder: MPEG-1 Layer 3 (MP3) (audio/mpeg, mpegversion=(int)1, mpegaudioversion=(int)1, layer=(int)3, rate=(int)44100, channels=(int)2, parsed=(boolean)true)ERROR: pipeline doesn't want to preroll.
The selected card (hw;Loopback) is handled by 4a, call close on 'multimedia'
Detected systemd unit file!
Port detected: 31002
ON-REPLY 1:ahl-4a/multimedia: OK
{
  "jtype":"afb-reply",
  "request":{
    "status":"success",
    "info":"Stream closed!"
  }
}