According to the Apple Macintosh Family Hardware Reference manual:
"The external sound jack is at standard line level (approximately 1.5
volts peak-to-peak) and its source impedance is approximately 47 ohms. The
jack is capable of driving a headphone load of 8 to 600 ohms, or the input
to almost any audio amplifier or amplified speakers. It will not
adequately drive a directly connected external speaker. The sound jack is
short-circuit protected."