Is the only way to increase the amount of wounds you have to take Sound Constitution? Or do you gain some amount of wounds when you increase a rank?

Yep, taking the Sound Consitution talent is the standard way to increase your wounds. You don't get any just for moving up a rank like in DnD.

Savage said:

That statement just made something click and the lightbulb went on above my head. I kept wondering why my players asked me at character gen if you add your toughness bonus to your starting wounds... They learned to roleplay in the DnD system.

It's an interesting thought though... adding TB to starting wounds.

Nah, the TB is already subtracted from nearly any damage you take. This would make it appear twice in your character's durability.