Questions about Serious Sam Complete Pack.

the artwork and the soundtrack should be inside your game's folder which is located at /Steam/steamapps/common/

Judging by the sub, it doesn't seem to be the Deluxe edition, but you can always check by checking the DLC you have for the game. It should include the Deluxe upgrade if you have the Deluxe edition.

The Complete Pack is different everywhere. I got mine off Amazon for like $10 and I had to pay for the deluxe upgrade, which was like $1.99 during a sale. You can tell off which DLC you have for Serious Sam 3: BFE and whether or not you have the non HD versions of the first two games. Some versions at other sites are incomplete, as well. In my case, I didn't buy a complete pack, it was called The Most Serious Bundle Ever, but most people would just list it as the Complete Pack, on here.

You will need to check, yourself. This is the stuff you should have. See if you have the non HD versions of the games and the DLC, then you will know if you have the Deluxe version or not. The extra DLC should be DLC3_Models and the Sniper Scope for Devastator.

The upgrade is on sale right now, so you might want to check if you want it, as it won't be that cheap forever.

I have the complete pack, looks like it is the deluxe version. Look in the tools "Serious Sam 3 bonus content".

Which Serious Sam Game it's most played online?

Even the classic ones are played today, I just started coop of Second Encounter this weekend with my pal, and some random dudes connected at once.
Dunno bout other modes, Sam is coop first for me.