If I am building on results of my earlier works, do I have to include them in my bibliography? In my master's thesis I am building on two earlier works of myself. 
-> One is a study project I've done together with two fellow students, -> the other one is an assignment I did on my own.

If I'm using results from these, should I include them in the Bibliography?

Arguments against including them might be: 


They are not published, a reader of my thesis therefore wouldn't be able to
look these sources up. (They don't qualify as quotable)
Also the effort involved in this works was much less than the amount
of work I'm doing for my thesis. (Some doubts on them beeing worth quoting)
And finally I am the author or one of the authors of these works. (I am not using foreign thought material)


Nevertheless it feels somehow incomplete to just leave them out.
Are these works available in any published form or any other permanent online form, such as a technical report? If so then cite. If not, then there's nothing to cite. In my opinion, it would look strange to cite a project done in some previous course.

What you can do is put an acknowledgment in the back of your thesis/paper to your previous "co-authors".