Hello, There might be an issue with your flash. We recommend you to check the version of flash and update it if it is an older version. You can check version the other solution here:-

https://support.mozilla.org/en-US/questions/970157

Hi dawoon, a couple other things you could investigate:

At the time that audio is playing but you hear silence:

Does Firefox display a little speaker icon on the page's tab up on the tab bar, indicating that Firefox is detecting audio playing in the page?

Assuming Firefox displays the audio icon:

Does clicking it make any difference? It operates as a mute/unmute button.

If Firefox thinks it is sending audio to Windows, but Windows isn't playing it, could you check the mixer?

On the Windows Taskbar, in the "notification area" at the lower right (sometimes called the system tray), right-click the speaker icon and choose Open Volume Mixer. Here you can see whether Firefox shows any volume or is muted at the system level. Anything unusual?

Note: If the site is using Flash, check for a muted Flash player in the mixer. When Protected Mode is enabled (the default setting), the mixer usually would show that Firefox is NOT sending audio, that the audio is coming from a separate Flash process.

Plug something into the speaker jack.