You're saying this as if something is wrong, but there is currently no browser that supports DTS audio. The best thing to do is just press play and trust that Emby is bringing it to you in the best way possible.
If you want these files to direct play, you'll need to go to your browser vendor and ask for them to pony up for a license to include DTS decoding in the browser. Or you can re-encode your files to use AAC audio instead of direct play, which is much more universally compatible.
Not really saying it as if it's wrong. As you know, I've just got rid of the FreeNAS box and set up from scratch with an unRaid box and all I'm doing is making sure my config is right and working as it should.
As to trusting that Emby is bringing it to me the best way possible - if I'd taken that approach I'd be watching transcoded playback on WMC. Once I've confirmed that my setup is in-fact set up correctly and is working as intended, then I will trust that Emby is serving it the best way possible.
Quite clearly I'm not going to my browser vendor any time soon to ask them to pony up for anything; I seem to have irritated you with what you thought was a stupid question - sorry for that, if i'd known the answer, I wouldn't have asked the question. I've tested in the past and browsers on the same LAN have direct played, just happens that I chose an AAC file to test with, rather than DTS/AC3.
Thanks for the info on audio streams causing browser playback transcode.