Can I hook up my stereo/boom-box's line-out (headphone) to my Mac's line-in (or microphone input)? What will be the side-effects, if any?
I want to record old cassette tapes as MP3/WAV/whatever on my Mac.
tl;dr: You can connect a "line-out" to a "line-in" and all will be well.
As slhck commented below, current MacBooks have a "line-in" socket that is for "a line-level microphone" which isn't your normal mic-in socket found on typical Windows/Linux PC hardware. A line-level input should be less vulnerable than a mic-level input (though it isn't designed for a headphone output expecting a 32-ohm impedance.) Designers of modern Macs and PCs probably make provision for end-users plugging inappropriate outputs into sensitive inputs
Normally, each of the following audio interfaces are different and (at least for quality reasons) should not be mixed up
Each interface is electrically different.
Line-in is explicitly designed to be connected to line-out (on a different device usually) and vice versa. These are high-impedance, low-current signal interfaces.
Microphone-in is designed for connecting microphones, these produce much weaker signals than line-out and normally need pre-amplification before being fed to a line-in interface.
Headphone outputs are designed for relatively low-impedance devices that use a much higher signal level. It would normally be a bad idea to connect a headphone-output to a microphone-input, at best you should expect clipping and distortion.
I think some computers have connectors whose electronics can be switched in software to provide appropriate impedance etc for two different signal levels.
Stricly speaking, you should use impedance matching and attenuation in between headphone-out and microphone input. This can be done with attenuating leads, DI boxes etc
For example, the description of this product says
Or this one
Sure, just do it. You said you're on a Mac, which means that there's no separate input for microphone and line signals.
Just plug it in, using a stereo jack cable …
… and select the appropriate input source under System Preferences » Sound » Input. You should level the input volume so that there is no clipping (i.e. the bars shouldn't be full all the time).
And now you need a program to record. I suggest Audacity, which is free, open source, and comes for all operating systems.
Yes, you can do it but it won't be as clean as taking your tapes to a professional and having them digitized.