One of the most important parts of a game is creating the immersion, that sense of being a part of the game. Things like bad level design, bad visual design and bad sound design can break that barrier and the player will not be able to connect with the game as the developer planned. I would argue that sound has one of the most important part to creating this sense of involvement within the game. Making sure all the surrounding sound effects are accurate are key to this. Sound is also a big part of level design as intelligently placed sound can help make the player progress in the preferred way through the level, it is an important part of triggered events.
If done well and successfully sound can help to create a sense of tension, stir emotions and even play on nostalgia.
Some games execute the tension very well through their sound; one game that stands out to me as doing this is Outlast. Even from the start up menu you have this horrible sense of tension and dread (even to those who do not know what they are about to inflict upon themselves.) Whenever I play this game with friends, the moment we hear the opening sounds we are straight back to feeling on edge and unsure about the whole situation. Even when in the middle of a level, you are constantly questioning what all the sounds are; some of the levels even revolve around the use of sound. For example, the guy we are avoiding at the moment has a chain that drags on the floor as he walks, so as soon as you hear that sound you know you have to hide as fast as possible. Then once hidden it is a question of whether it is your own breathing you can hear or someone else!
This game gets me every time!
Sounds also help to create emotions in the player; one too many games have had me bawling my eyes out and I can bet you that music in the game has helped achieve this effect. A good example of this, is in the opening of The Last of Us.
This opening is so well done as it established this personal relationship with the characters within the first few minutes of meeting them. In addition to this development the sounds and visuals support this well; when the music starts at the part where one of the characters is dying in the opening, it really stirs those emotions (makes me cry every time.)
Music from older games really starts to play on nostalgia these days, music from games we played when younger really stir our emotions when we hear them. When you hear a song from a game you played years ago it fills you with that sense of excitement and enjoyment you got from the first time you played it. Prime examples (at least for me personally) are the Legend of Zelda theme and the Elder Scrolls theme, whenever I hear these themes it makes me want to go play them all over again. When a song has this effect over you, you know that the games soundtrack was a very well done and successful one as it still remains in your memory.
My inner adventurer is crying with excitement!