Strange, never encountered that. I would try updating your video and sound drivers, if you haven't already.

Also, a tip for Homeworld 2 newbies... the enemy's starting force scales in relation to your own. This means the stronger your fleet at the end of a mission, the harder the next one will be. If you are having a very hard time beating a particular mission, reload the previous mission and end it with far fewer forces. It is often easier to scale up your own fleet to meet their size than to start out maxxed and have to face a monster of an enemy.