The Fantasy Grounds integration uses the web app, not the online application.
As at least one person is hearing the sound then you’ll need to check that the people who aren’t hearing it are: using the right invitation link to open their browser, after pressing the “start” button each player has allowed sounds for the Syrinscape website in their browser (you’ll normally see a speaker icon in the browser tab), the browser tab isn’t going to sleep - most browsers have a setting to save resources and put tabs you’re not viewing to sleep, etc…
Don’t regenerate the Web Player Session URL - only ever do this once for your FG install.
@ian.butler - in your next session, once all the players have joined share the invitation to chat and ask them all to click the link and press the “Start” button in the browser that opens. Once they’ve all done this, play a “music” sound to check they can all hear. As the GM open the “Join my Syrinscape session!” link from chat and make sure you can also hear the music.
Troubleshoot anyone who can’t hear the music - checking the things I mentioned above. The key thing to keep in mind is that everyone’s setup can be different - the browser needs to allow audio from the website (permissions usually accessing via a lock icon to the left of the URL in the browser), the local volume for the browser needs to be set correctly (i.e. not too low and not muted), the browser tab needs to be kept active to avoid going to sleep.