There are two ways to add custom tracks to amplitude, the easiest would be using Modulation, a free and open source GUI app based around Modulate by Adam Clixby.
Both methods assume you have the game files for Amplitude on your computer, obtaining the files from a real system is beyond the scope of the document, but if using RPCS3 they can be found in the dev_hdd0\game\NPUB31810\USRDIR
folder of the emulator.
Download Modulation from the GitHub releases page, you may also need the .NET 6.0 Desktop Runtime.