Yeah the STRM file basically shuvs the actual file in your player/browser so it is what you set as default audiotrack in your file. As for .mkv's, I started encoding and muxing them to mp4 instead of mkv. that format is more suitable for streaming and is more ocmpatible with hardware since it's more of a standard format. Some groups do a sloppy job in muxing to .mkv. They enable extra compression on it and some clients have trouble reading those files. Even google doesn't see those files as video files but docs. I noticed because some of my mkv's that were AMZN rips were transcoded and some had the blue document icon. MKV has it's advantages but not in the streaming area. You can mux in image based subtitle formats such as SUP and VOB but they are larger in size then text based formats such as SRT and VTT. Also, they sometimes require transcoding depending on what you play the file on. Text based formats are more comon and more widely supported. So in the end, MP4 is still the best option if you want to stream like a maniac. OH and for audio, I use AAC. DD and DD+ have their limitations where AAC has less and has a very optimal compression qulaity ratio. The biggest benefit is that it can support upto 48 channels of audio so is very suitable for those DTSHD-MA and TrueHD 7.1 audio conversions. AAC audio is also widely supported on browsers so if you want to play it back on that. So in the end, more then enough reasons to switch to compatible and optimized files.If you have/add multiple audio files, does the STRM codec default to the first track, the track set as "default" or the first compatible stream? I could see any one of those three options as working, but it would impact how I remux the files.
BTW, there is a web-based GUI for MKVToolNix in the user apps section. It works very well for either removing or adding audio streams, and marking them as the default stream. Works right on the server now without any downloading.