Hi-Fi sound is only available if you have a premium room subscription (paid room). If you have a premium room subscription you can enable this feature in the 'premium settings' area of your group setup pages.
Now click on the 'edit room' link in the 'my room' section.
Now click on the 'premium room settings' selection on the left side of the page.
Now click on the 'edit' button near the top right of the page, then click on the 'Hi Fidelity' selection.
Be sure to click on the 'save' button near the bottom right of the page when you are finished.
You will have to close and re-open your room after doing this if it is currently open.