Nominally it should be 700mV RMS ( 2V peak to peak) into 600R but often it varies from manufacturer to manufacturer, it can be 150mV into 1k to 5V ( BMW head ends).
Laptop audio out is normally meant to drive 32R headphones, so at max it will be around 4V P-P, with some pretty bad distortion. At a more sane level it will be 0.7V RMS, and use a load resistance of around 1k, so it will pretty much work direct into the audio side of the transmitter. Quality will not be the best, is it not possible to use an USB cable and an USB audio device instead?