Issue description

When running Firefox 58.0.1 (/nix/store/6sblfm52hkvzr2b4fp8y5bn8fvj2j6ag-firefox-58.0.1/bin/firefox) on a non-nixos Linux system (Debian stretch in this case), audio doesn't work. I can see the following messages on the console after running firefox and trying to play, for example, a youtube video:

[Child 14641, MediaPlayback #2] WARNING: 7fe5a381fd00 OpenCubeb() failed to init cubeb: file /build/firefox-58.0.1/dom/media/AudioStream.cpp, line 387
[Child 14641, MediaPlayback #2] WARNING: Decoder=7fe5aa798c00 [OnMediaSinkAudioError]: file /build/firefox-58.0.1/dom/media/MediaDecoderStateMachine.cpp, line 3642

Doing some searching, it appears that this error appears for people who aren't running pulseaudio, but I am running pulse via start-pulseaudio-x11 in my ~/.xinitrc (this is Debian's pulseaudio, version 10.0).

Steps to reproduce

1. Start non-nix-installed pulseaudio via start-pulseaudio-x11
2. Install firefox via nix-env -i firefox and run it
3. Try to play audio in firefox

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.9.0-5-amd64, Debian GNU/Linux, 9 (stretch)`
 - multi-user?: `no`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.16`
 - channels(jad): `"nixpkgs-18.03pre127228.91eb41b1383"`
 - nixpkgs: `/home/jad/.nix-defexpr/channels/nixpkgs`