The speakers you have are expecting 6 discrete signals (5.1 audio). That’s not encoded in any way. So it should work, although it seems pretty common that people are having issues with this.
HDMI can pass uncompressed 5.1 audio, but you’d need something on the other end to play it back or output it as 6 signals to your speakers.
If you are using optical you’d need 5.1 Dolby Digital or DTS to compress the 6 audio channels into a digital signal small enough to fit on an audio coax or optical cable. There is technology to take 5.1 uncompressed audio and encode it as Dolby Digital in real-time called “Dolby Live”.
SoundBlaster (remember them?) has an external USB sound card that can do 5.1 discrete output or 5.1 Dolby Digital encoded audio over optical. I don’t think the browser is going to be any better about passing the 5.1 audio to this sound card than it is doing to your onboard sound card though.