No audio playback in Youtube downloaded video files

Just installed Kodi 18 (June 17th build), but that was the first one I installed on my Windows 10 NUC PC, so I can not say if an older build worked. Everything plays brilliantly, except for any Youtube downloaded files (which I use JDownloader2 to download). The video plays fine but there is never any sound. It shows up as AAC but also shows up as UND under language in Kodi. It is not a problem with AAC in general because every non-youtube video I have with AAC plays fine. Also those YT videos play fine outside of Kodi on the PC, so I have the correct codecs on my PC. This is the only annoyance stopping me using Kodi as my dedicated player, as I download a lot from YT to watch offline. My old Dune HD player can play them fine, but I want to ditch that now. Any ideas ? Great work on everything else guys.

Here is a royalty free video clip for anyone to test that (it should have music playing) was downloaded from YT that won’t play any sound (same as others) : http://www.storespace.net/royaltyfree.mp4

Below is a MediaInfo snapshot of what the audio info says………..

Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 1 min 15 s
Bit rate mode : Variable
Bit rate : 126 kb/s
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 44.1 kHz
Frame rate : 43.066 FPS (1024 spf)
Compression mode : Lossy
Stream size : 1.13 MiB (10%)
Title : ISO Media file produced by Google Inc.
Encoded date : UTC 2016-08-12 23:39:20
Tagged date : UTC 2016-08-12 23:39:20