Mumble positional audio not working by default

**Game information**

All games that support positional audio in Mumble.

**Distribution name and version where applicable**

Arch Linux

**Flatpak info**

```
flatpak --version
Flatpak 1.12.2
```
```
flatpak --gl-drivers
nvidia-495-46
default
host
```
**Problem description**

Mumble positional audio doesn't work by default with flatpak steam. To fix this the user can run `flatpak override --user --device=shm com.valvesoftware.Steam`. I'm not sure what the security considerations are for this but it does solve the issue.

**Does this issue reproduce with native Steam**

No