On a RokuTV, you should be able to set both the Sleep Timer and turn off the display in order to listen to just audio. (unsure if available on all models of RokuTV).
To enable both the Sleep Timer and the Display Off, after selecting your content, press the * button on your Roku Remote which will bring up a "TV Settings Options Menu". You should see "Sleep Timer" and "Picture Off" options (or similar terms depending on manufacturer).
On a non-Roku TV (regular TV or smart TV with an HDMI attached Roku device), both of these features are going to depend on the features of your TV. The settings mentioned above for RokuTVs are not available on Roku devices.
Best you can do if you do not have external speakers is probably to install/use a dark screensaver on your Roku device and set your TV's Sleep Timer if supported by your TV. However, if you had the Roku Streambar, should be able to turn off your TV screen and still be able to stream and hear the audio. In either setup, probably a good idea to enable Bandwidth Saver on your Roku device to prevent the streaming from continuing after 4 hrs. (Settings/Network/Bandwidth Saver/ [set to On]. Otherwise, your Roku will continue to stream nonstop after your Sleep Timer on your TV has activated.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102