If the client audio is not set with these configurations, the host, Linux Mate, loses audio when the client VM is running. When testing the host’s audio while the client VM is running, the audio settings screen hangs. Once configured with the proper settings, the host and client functions as expected.

Going from VirtualBox 6 to version 7 I noticed the theme went from light to dark, so I thought there was a theme setting. I searched and could not find a theme setting.

I Google’d and found this provided by the Google Artificial Intelligence (AI):
In VirtualBox 7.1, theme customization, especially for dark mode, is platform-dependent: on Windows hosts, you can force dark mode through VirtualBox preferences, while on Linux and macOS, VirtualBox follows the system’s look and feel.
Here’s a more detailed explanation:
Windows Hosts: You can force a dark theme in VirtualBox 7.1 on Windows hosts by going to VirtualBox Preferences and enabling the dark mode option.

Linux Hosts: VirtualBox 7.1 on Linux follows the system’s theme, and it’s difficult to force a specific theme due to the variety of desktop environments and window managers.

macOS Hosts: VirtualBox on macOS also follows the system’s theme, making it easy to switch between light and dark modes.

Qt6 and GTK2: VirtualBox 7.1 is built on Qt6, which doesn’t have official support for GTK2 themes anymore, but VirtualBox integrates an old GTK2 support plugin for Linux, which requires exporting the environment variable QT_QPA_PLATFORMTHEME=gtk2.

My current Ubuntu theme setting is dark mode:

I hope this save someone time in a wild goose chase. All the examples I found were of users demonstrating VirtualBox 7 on the Windows OS.

When making changes to the VM, best to have the VM off. Make the changes and boot the VM.

This took me hours to figure out, with most of the time used searching and with no avail. So I started to experiment and found the answer.

In the Proxmox PVE VM, go to Hardware and make sure that Display is set to SPICE (qxl):

This will activate the audio in Windows 11.

Even though the audio is activated on Windows 11, unless the Audio output mode is set to Local on the RDP client, Windows will shows no audio.

Windows RDP client:

If using Remmina: