Of course, a mandolin is not just a high-pitched guitar. It has pairs of strings. its own size & shape, and it has its own timbre.
You might want to look into
MIDI. With MIDI you can synthesize almost any instrument, or a whole orchestra if you wish. It can do everything except the lyrics. Most of the background music you hear on TV or in current movies is MIDI. In the old days you had Henry Mancini and His Orchestra... Now it's MIDI. (Audacity does not support MIDI.)
A MIDI file doesn't contain audio... It's just the notes & timing (and some additional information... Something like sheet music for the computer, so the computer knows what instruments to play and what notes to play.
From what I understand, it's hard to get realistic guitar with MIDI, so mandolin may be "difficult" too. But, you should be able to find a decent virtual instrument, especially if you are willing to pay for it...
There will be a bit of a learning curve, but if you want to get a basic idea of what can be done with MIDI, just search the Internet for some free MIDI files, download them, and play them. Your soundcard/driver has a basic MIDI synth and Windows Media Player can play them, so you can play them on your computer without any special software or hardware.
However, it does take special software (a MIDI sequencer or a DAW) to compose & edit MIDI files and to render them to audio (WAV, MP3, etc.). Most MIDI music is created with a MIDI keyboard and then edited on a computer, but it can be done entirely "in the box" (on a computer).