First time I've heard of Bandlab, will need to give it a dive at some point!
I love the slow beginning to Sad - it takes its time in introducing various elements and really swelling to fill the mix. Great work with the orchestration here, and the choice of progression is well done. Nice and evocative, befitting of the title.
Happy has a rather interesting beginning - the percussion off the bat does wonders for setting the tone. Once again the gradual layering of different elements here works really well. The choice of mallets is great - deserves more love in general! The woodwind and brass accompaniments behind are great too, trading off melodies and never crowding the mix too much. As a whole this track is nice and thoughtful; akin to a soft kind of happiness and contentment.
The opening crescendo in Memory is gorgeous. The sparse use of percussion just to lay down a fundamental rhythm is a nice touch and gives the track a pulse so that the leading instruments can take a little bit of a less-is-more approach and not feel the need to fill out their melodies completely. As it progresses, I found this track to be rather pensive. I can't quite pin a happy or sombre mood to it - I feel like it is a little bit of both, honestly.
The choice of synth voicings in Relax is great - does a nice job of complementing the lead melody with a pseudo call-and-response kind of vibe. Again this is a nice pensive piece but it feels a little more definitive in its direction. Moving toward acceptance or perhaps a hopeful determination.
Great work, I enjoyed giving these a listen! I think they're super solid and that the execution of the theme was on point. :)