From Valve Developer Community
You can export an SFM session as either a two-dimensional movie (.mp4) or a still image (.tga) file.
To export a movie:
- On the File menu, click Export, and click Movie.
- Set the options in the Export Movie dialog box, and then click Export Movie. You can take the defaults until you know the options you want to modify.
Note:Depending on the complexity of your movie, the export process can take a while.
You can also export sound tracks, either as a group or individually.
To export sound tracks:
- Make sure the check boxes for all sound clips and tracks you want exported are selected.
- On the File menu, click Export.
- In the Export Movie dialog box, change the Export drop-down from Movie to Sound.