One does not have voltage divider action between two resistors unless there IS current flow. It is the current flow that creates the voltage divider action, baby!

150 ohm plus 1 ohm (total: 151 ohms) fed by 120vac would dissipate almost 96 watts. Scaling up by a factor of ten would reduce this to 9.6 watts, more do-able in a small package. Further scaling may be of benefit, depending on the line-level input impedance of your sound card. The line level input impedance of the sound card will set an upper bound as to the resistor values of the divider. Power dissipation will set a lower bound.

Perhaps protection such as back to back zener diodes of several watts rating across the sound card inputs in case the low ohm resistor opens should be considered. Paralled LEDs of reversed polarity is another possibility IF the large resistor in the divider is high enought to limit the LED current to a safe value. The light would indicate an overload condition.

Of course, the input impedance of the line level input of the sound card would have to very high compared to the paralell value of the voltage divider resistors or connecting the sound card will significantly affect the voltage divider results. When dealing with measuring the AC line voltage, some failure analysis should be done.

Computer based sound cards (ones that are contained entirely inside the PC, as a plug-in card on the motherboard) have voltage refferences at chassis (case) ground, which is connected to the gound conductor in the three conductor power cord and then to earth. This is not quite the same as the voltage on the neutral wire, which is what the hot AC lead is refferenced to.

To get around this, you can do the dual divider circuit as Soundhound suggested. Hook up one divider between the hot and gound, the other divider between return and gound, with the small (low ohm) resistor at the ground end.

Have the output of the hot divider go to the right line level audio channel and the return divider go to the left line level audio input. Make certain that the gain (volume setting on the sound card) is the same for left and right channels. The best way to do this is to set the gain (volume) to max and change the voltage divider ratio (both voltage dividers must be the same).

Then, have the software subtract the left channel signal from the right channel signal, the resultant waveform will be the hot signal relative to the return signal, what you are interested in. Then you can do FFTs and other analysis on the waveform.

OOH! Pretty pictures.

Paul