Thank you for your inquiry. Currently, switching sources on a particular SoundTouch system is not possible through the Alexa skill. However, changing SoundTouch presets certainly is. For a full list of currently available commands, please see our latest release notes.
In regards to changing sources within the SoundTouch app itself; which system are you currently using? If you are referring to a SoundTouch TV or home theater system, it is not possible to change input sources (TV/CBL-SAT/DVD-Blu-Ray/etc.) within the SoundTouch app.