The importance of soundeffects in a game

Recently I heard on radio about a student in game-development doing his exam paper on the importance of sound in videogames. Not surpringsingly he had drawn the conclusion that sound effects are very important. For example, when walking in different rooms, the sound of walking should match the sound you would actually hear in that environment.

For Rolling Coaster, I started adding sound rather late. But when I found decent sounds for events in the game, the game was improved a lot. Try looking at the following shot without sound, it won't give you the same feel of presence as it would with the sounds currently used.

Now, I'm not saying that I've used perfect soundeffects for everyting here... it's just that the sounds are way better than not having any sound at all! Also, there is a Doppler-effect on the frequency that really makes a huge difference (which is really simple to add, see wikipedia for the theory)