I can think of three things to check.

Is Valheim some how muted or turned way down in your volume mixer? Open Valheim, then open your volume mixer and check. Right click the speaker icon in your taskbar to open this.

Are you using the wrong sound option for your system in the sound settings? I've seen this happen before, somehow it got set incorrectly, but once I switched it back to my default it was fine. Go into sound settings, then look in the app volumes and device preferences while Valheim is running. You'll see the settings in the drop down boxes. It may have been changed somehow.

Check your antivirus, I've had them do some really weird stuff to games and knocking out the sound wouldn't surprise me. Make sure you're in the exceptions for the game. Do the same with the firewall.