http://audacity.sourceforge.net
Audacity is an excellent freeware audio editor.
It's not a sequencer, though. For sequencing, I use a commercial app called FL Studio.
http://flstudio.com . The cheap versions don't do much--I use the "Producer Edition". There are freeware sequencers available. There's also a type of program called a "tracker", which is a grid-oriented sequencing promgra. They take some learning, but can do what you want. I don't have one to recommend, but google "music tracker", you'll find something. Many years ago I used a tracker called Renoise, and it was okay.