https://www.theverge.com/2019/5/7/18535081/alexa-windows-10-hands-free-wake-word-push-to-talk-update

Alexa can now be summoned with just your voice on any Windows 10 PC. The latest version of the Alexa app on the Windows Store notes that Amazon’s voice assistant will now listen for the Alexa wake word “when the app is running in the background or minimized.” The previous version of the app was push-to-talk only. Also included in this update is support for music playback from Pandora.

Alexa’s Windows 10 app has been available since last November but at launch only a limited number of Alexa Built-in PCs allowed users to wake Alexa using just their voice. Otherwise you’d have to press a button to have Alexa listen for your command. The push-to-talk option is still available, and you can switch between the two modes in the app’s settings.