I'll add myself to the long list of 'CS757 III No Cockpit Sounds' victims. Apart from the engines, I am getting no cockpit sounds. ALL other aircraft work perfectly: another company, FSLABS, MAJESTIC, another company, TFDI, A2A, another company, etc.

Platform: P3Dv4.5
Version: Prepar3D_v4_Professional_4.5.14.34698

CS 757, latest version 1.5

What I've tried to do so far:
Uninstalled all CS757 packs
Reinstalled Basic Pack with all AV (including Windows Defender) turned off
Repaired Basic Pack with all AV (including Windows Defender) turned off

dll.xml  -  checked
Path: C:\Users\User\AppData\Roaming\Lockheed Martin\Prepar3D v4
confirmed entry:
  <Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
  </Launch.Addon>

cs.sound.dll  and  cs.panel.sound.dll   -   checked
Path:  D:\Lockheed Martin\Prepar3D v4\Captain_Sim\bin

Computer Specks: see attachment
Computer Audio:
           Sound Cards
                 NVIDIA High Definition Audio
                 USB Audio Device
                 NVIDIA Virtual Audio Device (Wave Extensible) (WDM)
                 ASUS XONAR SC150 AUDIO DEVICE
                 Realtek High Definition Audio
           Playback Devices
                 Realtek Digital Output (Realtek High Definition Audio)
                 BenQ XL2410T (NVIDIA High Definition Audio)
                 BenQ BL2480 (NVIDIA High Definition Audio)
                 SPDIF Out (2- STRIX SOUND CARD)
                 Speakers (2- STRIX SOUND CARD)      (default)
                 Speakers (Realtek High Definition Audio)
                 U32J59x (NVIDIA High Definition Audio)
           Recording Device
                 Microphone (SteelSeries H Wireless)

So I'm open for any suggestions.

Thanx
Ron

I just spent almost a whole day going through everything suggested in the NO SYSTEM SOUNDS messages.

With absolutely 'no joy'!

But here is what I found that got it to FINALLY work:
I am not an IT guy, so don't know any xml entry rules - I can just copy and paste

So what was missing from your info is: when editing the dll.xml file and adding the following:
<Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
</Launch.Addon>

it seems to be vital to end this with the following line
</SimBase.Document>

I found out about this after running tis fix-it file (which doesn't include P3Dv4 btw!)...
dll-xml-cs-sound-fix.exe
...and comparing the dll.xml from P3Dv3 with the one from P3Dv4
That's when I noticed that the </SimBase.Document> line was missing and since adding that it's all good now. It just took too long to get there.