You can access the source code of ModPlug Player and all of its components on the project page
https://github.com/ModPlugPlayer
ModPlug Player uses Andromeda Signal Lab components, you can access the source code of Andromeda Signal Lab components on the project page
https://github.com/AndromedaSignalLab

To download all the source code of ModPlug Player and its components, you can use the command
git clone–recurse-submodules https://github.com/ModPlugPlayer/ModPlugPlayer.git ModPlugPlayer

If you want to download a specific version of source code (for example Alpha 5 release), you can use the command
git clone –branch Alpha5 –recurse-submodules https://github.com/ModPlugPlayer/ModPlugPlayer.git ModPlugPlayer