LMMS does what FL studio does for free.
There is also the very powerful
Ardour, considered the best DAW, probably overkill for most users.
Audacity can be used for simple multi track audio editing, it can be extended with gazillion filters.
As for video there are many, such as
Openshot or Virtualdub. You just need to properly search for Free and Open Source projects, even if you are still chained to a proprietary operating system...