r339929 no audio output to some sound devices with Firefox and some other applications

[Graham Perrin]

<https://vimeo.com/130130394> for example:

- audible in Chromium, Falkon and Web
- silent in Firefox, New Moon (Pale Moon) and Waterfox.

HP EliteBook 8570p, integral loudspeakers.

If I recall correctly: yesterday I _did_ get audio from at least one of the affected applications, whilst the notebook was docked. A Philips display with integral loudspeakers at one of the two DisplayPort sockets at the rear of the dock.