Practically everything, down to the motherboard. The most common upgrades would be GPU and RAM (and PSU if necessary) about 2-3 years in the future. Then a CPU and storage upgrade about 5+ years in the future.
That is standard for every PC, it is recommended for better audio quality both in and out. You will need something like this: https://www.pbtech.co.nz/product/CABSTT1721995/StarTech-MUYHSFMM-35mm-4-Position-to-2x-3-Position