The XWM file format is an audio format used by a number of games, such as Space Engineers, Fallout 4, and The Elder Scrolls V: Skyrim.
It’s official name is the xWMA format, and support for using it in these games comes from Microsoft’s XAudio2 API. XAudio2 is the successor to DirectSound, and according to Microsoft is “primarily intended for developing high performance audio engines for games” — which is probably why you don’t see or hear much about .xwm files outside of game modding communities.
.XWM files are actually RIFF containers, which hold the xWMA audio. If you open an XWM file in a hex editor, you’ll see that the first four bytes are ‘RIFF’. Bytes 9-12 give the audio format, which will be ‘XWMA’ in ASCII.
RIFF is a very common format used by WAV and AVI files, the much newer WebP format developed by Google, and many more.
Creating XWM files
If you’re reading this, then you’re probably interested in creating your own XWM files to add to a game. The easiest way is to use one of the many third-party converters available. This saves you having to install any SDKs or manually set command line options. Most of the converters are labelled as “For Skyrim”, but they should all work for any game using XWM audio. I recommend MultiXwm which does the job nicely.
Once you’ve downloaded MultiXwm and extracted the zip file, start it up and hit the “Add File(s)” button and load in your source files. Make sure “aduio->xwm” is selected under type, leave the bitrate at default, and then press start.
After a short while it should have processed all your files, and they’ll be sitting in a directory called “output” in the MultiXwm folder. You can then add these files to your game.