Driving line input of a speaker from a differential amplifier, why is the output clipped?
Step 1:

PC speakers are crappy at best. Who knows what they call ground, how they relate it and where exactly it comes from.

Thus, Step 2:

You need to have ONE ground point in your system.

Which follows to Step 3:

You need AC coupling in your schematic, because you do not know what will be at the output, in terms of power-loops through whatever other connection, such as power-earth.

Gives Step 4:

The same ground loops may be causing your input signal to jump/drop to either power-rail and op-amps only really work with voltages between their power-rails. Many op-amps even only work with a (small) part in the middle of that region.

Making your schematic:







simulate this circuit &ndash; Schematic created using CircuitLab

Where the 5k resistors make a voltage divider to keep your incoming signals on average around 1/2 Vcc, as long as your signal source can cope with 5k/3 =~ 1.6k loading that should be fine. If your signal source is very weak, you may need a balancing resistor coming from a dedicated 1/2Vcc point, like so:







simulate this circuit

Whether your capacitors need to be in the range of single nF, hundreds of nF, or several &mu;F, only someone knowing all the specs of the signal (and allowable star-up behaviour) can tell you.