We've had a number of queries recently from customers using the mu-so as a 'soundbar' - feeding it via S/PDIF from either a TV or directly from the S/PDIF output of a set top box such as a Virgin Media or Sky receiver.
The mu-so - in common with our other streamers - does not have any form of multi-channel audio decoding and so expects a regular PCM stereo audio input. If fed with bitstream Dolby Digital or DTS then the unit will either output 'noise' or will be silent. The source device must be set up to output PCM stereo audio to obtain audio from either the mu-so or one of our streamers and it is in this instance that we have had queries over volume.
Th reason for the reduced volume is that when a soundtrack is supplied in multichannel and the Set Top Box or TV has to convert it to 2 channel stereo it has to mix the additional centre, surround and LFE channels into the stereo left and right without digitally clipping.
If - for example - the front left, surround left, rear left and centre channels (if you have a 7.1 source) all have a 0dB (full level) signal on them then when they are mixed to give the stereo left the resultant signal would be above 0db. As 0db is the highest level that can be represented in a digital audio signal then the output would be driven into clipping. (Clipping digital audio signals are not at all pleasant!) To avoid the possibility of clipping whilst keeping processing to a minimum, all channels are therefore simply attenuated before being summed together, however this results in the overall volume of the stereo mix decreasing, normally by around 10dB.
On a proper AV processors the downmix headroom needed is calculated by the processor which then does the mix down and then boosts the signal back up in the analogue domain using volume control chips and hence mixes down to correct levels and gets optimal signal to noise.
This loss of level is an attribute of the device doing the downmixing (the TV or Set Top Box) and is not a problem or fault of the mu-so or streamer.