The best replacement for stereo is mono.

The best alternatives to stereo are rated as following:
★★★★★  mono
★★★★★  joint stereo
★★★★  surround
★★★  surround sound
★★  5.1
★  stereo
★  compound microscope
★  mono sound
★  mono recording
★  mono cable
★  multichannel fraps
★  surround headset
★  multichannel
★  sap
★  surround headphones
★  penny
★  home theater
  mono better
  dolby digital
  surround sound headset
  mono jack
  dual mono
  joint
  mono cables
  radio
  av receiver
  joint stereo audacity
  4 channel
  dual channel
  4 ohm bridged
  balanced
  plain stereo
  stereo direct
  mono beatles
  mono vinyl
  matrix
  dolby
  j-stereo
  mono headphones
  mono music
  normal
  virtual surround
  normal stereo
  mono audio
  bridged

Your alternative for stereo is not listed? Suggest!
Speaking about stereo, one probably shouldn't forget about aux input.

Other search terms related to stereo and their ratings are:
★★★★★ aux input
★ ipod connection
You have in mind a companion for stereo and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of stereo in all years. Click on graph for more.
stereo
mono
joint stereo