Low microphone level 5070 ThinOs 8.6_019 RTME 2.5 Skype for Business

We have a problem with a 5070 Thin client with ThinOS 8.6_019 with RTME 2.5 for Skype for Business. The volume of the microphone is very low and almost unintelligible. We also tested with RTME 2.4 and various other models of Dell Thin clients but de issue remains. If we place a checkbox in the Citrix session by the option within RTME to Adjust Microphone Level, the volume of the Microphone is louder but it takes a while for the volume to reach an acceptable level.

We also have adjusted various settings within WMS and locally on the thin client, but this also did not matter.

We both tested with a Jabra Headset and a Plantronics Headset but with both headsets the issue remains.

Is someone familiar with this issue and how can this be solved?