Basic ideas:
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Game sounds
  Generally, should be set with sound on during attract mode, unless obnoxious.
    Exception: A room used for multiple purposes where you want the games left on.
  You may want to set all your games to a general volume level.
  Be aware that the sounds may disturb other nearby portions of the house.
  Remember sound insulation if building a new gameroom.

Music
  80's, obviously. Don't forget Pac-Man Fever!
    Hungry Like a Wolf, Mr. Roboto, etc.
  Jukebox, but may be expensive, and limits your selection.
  MP3 player. A PC in a MAME style cabinet would be ideal.
    Gameroom jukebox: http://www.madville.com/comments.php?op=main&aid=5982
    Arcade jukebox 6: http://home.att.net/~mark.schwartz/

Sound Stage with System
  Not arcade like, but gameroom like.
  Semi-professional amplifier, speakers, microphone.
  Can be used as a PA system.
  Can be used as a main stereo system.
  Can be used as a karaoke system.
  Example: http://www.carvin.com/cgi-bin/Isearch.exe?CFG=2&P2=620-822&P1=SYS1

Artificial Arcade Ambience
  Pipe in the sounds of a larger vintage arcade into your own.
  http://users.adelphia.net/~hofle/arcade/