Flawless! I almost thought it was a native Linux app! But as OP states, no audio, for me either. Here is my log!
steam-374040.tar.gz

https://gist.github.com/daasuharo/236148034fd15bd354eda262212ea607
Proton 3.7-5
Concur with @BrosephJ and @BrandonDyer64 except Ubuntu's sidebar pushes game window a little offscreen to the right.
DS4 controller works perfectly.

Looks like this is a problem with the buildin library "xaudio2_7".

Got sound to work by installing "xact" with winetricks and then setting "xaudio2_7" to native with winecfg.

With Proton version 3.16-5 and FAudio the game starts up on first start and with audio. But it crashes every time the moment when pressing any key before the main menu.

Systeminfo: https://gist.github.com/NoXPhasma/c69b8988a35383bc6f9a452872dbdfb2
Proton Version 3.16-5
Proton Log: steam-374040.log

Does it require FAudio to be compiled with WMA support?

When I run with debug and FFMPEG enabled I get this error:

WARN: 

Assertion failure at FAudioVoice_SetOutputMatrix (/home/noxp/build/FAudio/src/FAudio.c:1519), triggered 2 times:
  'i < voice->sends.SendCount'

Logfile: steam-374040.log

It doesn't look like a FFMPEG related problem, I guess @flibitijibibo knows more.

Only found one thing in the log, which should now be fixed:

https://github.com/FNA-XNA/FAudio/commit/13429301cccfe7a051a7f3e2e511d8fbe0fd96bb

Someone should probably tell them they have an invalid call stream...

That commit fixed it. On exit of the game another debug warning raised, but I guess that can be ignored. But I want to show it anyway:

WARN: 

Assertion failure at LinkedList_RemoveEntry (/home/noxp/build/FAudio/src/FAudio_internal.c:236), triggered 1 time:
  '0 && "LinkedList element not found!"'

steam-374040.log

I've also filed a bug report to the Portal Knight developers to let them know about this issue.

Thanks! We got the bug report and we will take a look into it. Note that we currently can't really support the Linux 'version' of the game but it's cool that it seems to run nicely. /jkoenen (works on the engine used in Portal Knights)

Portal Knights is working great, no tweaking of xact needed anymore. I did have a problem starting it up--in portal_knights.cfg, I changed "window Type": "Default" to "Fullscreen" to get it to start, otherwise it would crash. (Error in the log was that the Display wasn't found.) Once I changed the value once, it started right up. I'm playing in windowed, but the portal_knights.cfg file says "Default" again for windowType. Is portal_knights.cfg set elsewhere than in the game's directory? (It's playing in windowed mode, but I'm not sure why when the config file isn't reflecting it.)

I also needed to use local libraries, in order for gnutls to work.

In the interest of testing, I tried Portal Knights on another user account. I had to kill the process once, but it started fine on the second try. No issues with incorrect window types in portal_knights.cfg. I'm thinking it's a bug with the game, since there were windows users having the same issue.

I'm trying to mate it work on proton 5.0. PROTON_USE_WINED3D, delete winepulse.drv, manual create portal_knights.cfg, never worked. I get only a yellow cursor, audio, and a dummy window. Maybe creating portal_knights.cfg in a windows environment?

https://gist.github.com/marcbelmont/1f264939e3d3120d094fa4e6aedea8d6 If anyone's having problems starting it with a blank portal_knights.cfg file. Someone kindly uploaded theirs. (Whoever it is, thank you!)

steam-374040.log
Here is my log, I would switch to experimental instead of 5.0-10 but it's currently stable and I actually want to play the game. I'm having to use Windowed mode, hopefully this helps.