I'm a musician (Cubase user)... among other things. ^_^
It is correct that most sound cards doesn't handle several sources well; especially integrated solutions, but that shouldn't be an issue here, as the browser should be considered 1 source.
What I don't get is; your game is hosted on a server, but I'd think the game is executed client-side, while the server just delivers the play-files? I mean - your sound card on the server (and the graphics card) should be under 0% load each, while your hard drive and probably also the CPU, will be utilized some, to keep everyone in sync etc...
I don't know any game where the server plays the sound, then sends the wave audio to the player's PC... o_O
Each client can and do play their own sound.
Originally, the server/client was going to be one and the same. So that friends can host and play together easily. After a few limitations I ran into, as well as design changes, I expanded on the idea to allow a dedicated server to run in a shard-like manner.
All being the same client/server.
It looks like im going to scratch that and simply keep things under my control, hosting multiple servers if needed. The shard method looks like the best way to divide resources and keep server performance at a maximum. At least for the larger servers.
My game is a small-scale mmo In a way because my world is planned to scale nicely for small groups, and large groups of players. So far things are open world with portals between world areas(server shards)
I might come back later to optimize the local server/client to only allow small groups of players. But for now, large scale is my first and primary focus.