Well, I tried to purchase LPU 12 mp3s, but it is out of stock.
I found out that the majority of musics are available, but there are some digital musics saying they are ‘out of stock’.
I don’t understand how could a digital content could be out of stock.
But most of all, I really want to buy those soundtracks.
Please solve the issue 
You ask a valid question. 
That site is pretty broken, it might mean physcial copies as opposed to digital ones. My advice is to break out your eye patch, peg leg and hook.
Maybe contact LPU HQ about it? I guess this could be fixed easily
Yeap. I just sent email to them. Hope they would reply me soon :0
I was going to suggest that you can at least stream the album if you have a membership, but I’m not entirely sure if that is possible right now because I need to renew. Please let us know if this gets fixed or not, going to move this into #help-site-feedback