I'll just throw some possibilities at you.... All of this is optional, or it depends on the particular circuit design...
You'll almost never find a resistor in series with the input, so we'll ignore that possibility.
You might find a "protection resistor" in series with a preamp-output, or headphone-output. It's purpose is to limit current (and prevent damage) if the output is accidentally shorted to ground. A preamp (designed to drive loads of 10K & up) might have a 1k resistor. A headphone amp might have a series-output resistor of 20 Ohms or less. The higher the resistance the more in interacts with the headphone impedance (which is not uniform across the frequency spectrum). I've seen one headphone design with a current limiting resistor is inside the feedback loop. That creates a low effective output impedance (for flat frequency response) while still limiting current when the amp clips or is shorted. (When an amplifier clips, the negative feedback no longer has any effect.)
You won't find a resistor in series with the output of a power amp. A power amp needs to deliver maximum power to the 4-Ohm or 8-Ohm load. If a power amp has short circuit protection, it's more complicated.
A parallel resistor in parallel with an input or output to ground is usually there to "pull" the output to ground when nothing is connected, or when something with a series capacitor is connected. This is usually 10K - 100K, but on the output (especially the output of a headphone or power amp), it might be around 1k. If the input or output is capacitively coupled and one end of the capacitor is not connected to anything, the unconnected end can "float" up or down. Then if you connect something that suddenly pulls it to ground, you'll get a "snap" or a "pop". I know of one case where a soundcard-input was floating-up during recording and it started to distort after about a minute. In this case, I think it was whatever was connected to the soundcard causing the problem, but a resistor to ground at the soundcard input solved the problem.
Pro microphones are low impedance. On a preamp, there is usually a resistor of around 1K in parallel with the (differential) mic input. That resistor properly "loads" the microphone (for the proper sound/frequency response), and a lower impedance also reduces noise pick-up, especially when no mic is connected. On some high-end preamps, this input resistance is variable.