You can often find the answer to why something is available in Debian, and not in LMDE, by having a look at PTS. For LMMS the story is here: http://packages.qa.debian.org/l/lmms.html
. Basically, LMMS was removed from Debian testing mid last year, and only two months back was it migrated from unstable back to testing. Inbetween it has not been available in testing. Since LMDE is currently at UP#4, which was released before LMMS was accepted back into testing, this explains why when using LMDE latest with UP#4 you can not find LMMS. Currently UP#5 is being working on, expected to be available on LMDE latest by end of August. LMMS will then be available on LMDE again.
As for Oracle Java (formerly Sun Java), Oracle has revoked the license for operating systems to distribute Oracle Java. See Oracle retires licence for distributing its Java with Linux
. You'll have to install it manually from the Oracle website if you can't make do with OpenJDK.