I've found the best way of doing atmosphere isn't sound, or voices, though they help, but is to make the NPC's act sensibly and with motive. For example, why on earth would 3 goblins attack 5 guys, that makes no sense. So I give them a net, and a bounty reward for catching dwarves. Of all the feedback I get about my GMing, it's that my NPC's make the world seem realistic, since each has a motive and isn't NPC#2361 who has knowledge of items 14-23. It's the people, not the mood or the visuals that create that spark of life and wonder in a setting. They make you sit down and wonder how Officer Chocolate became a security guard for this corporation, who is this brother he's talking about, and why is he eating 3 foot long subs at 2am. Remember that it is the through NPC's that the players should interact with the rest of the world with, and I'm not talking about Gods and Demons, but the common folk that they see on the street. A good recommendation if you're not on the spot creative, is to make a list of 100 or so names, and write 3-5 adjectives next to each one, and select one. That way when a player talks to them, the NPC has a personality. Of course you should vary this based on location and events going on in town. Another thing that always has been given me praise for my settings is the sense of scale. I never scale enemies based on the party ability. I have a normal range of very easy to very hard for encounters, and I expect the players to run if they're fighting something stronger than them. The world doesn't fall into your lap how you like, you're never gonna fight stuff easy every time, you have to earn what you want. So as the players get incredibly strong, I still throw common thugs at them, which can see them sent to the jailhouse fast if they just openly murder them. Guards don't favor you because you're a PC. If your go to solution for every problem is just to stab them, then you will get arrested eventually, like in real life. Exceptional individual or not, you're still bound to the laws of society. This I have been told is one of the best things about me GMing since it adds a facet of realism. Also I hate murderhobo, so it keeps those kinds of players out.