There will be v0.4.3 for Ubuntu 14.04?

As 14.04 is an LTS release the packages are rarely updated. LTS releases concentrate on what works and is tested. It is rare for LTS releases to keep bang up to date as frequent changes do not occur.

If you report a bug for the package and state the bug and security fixes in the new version. If they are significant then the package will be updated. Ubuntu in general will not update a package simply because a new version is out. It is not a rolling release OS.

In v0.4.2 (Ubuntu 14.04):
When I try to launch Linthesia from the Terminal, I'm taken to a folder to navigate to a MIDI file, but it crashes immediately after selecting a file. Terminal:

milzou@asus ~ $ linthesia

(linthesia:24030): GdkGLExt-WARNING **: cannot load PangoFont

(linthesia:24030): glibmm-ERROR **:
unhandled exception (type std::exception) in signal handler:
what: std::exception

Trasovací/ladící past (SIGTRAP)

File a bug and request SRU if you want it fixed in Trusty.
https://wiki.ubuntu.com/StableReleaseUpdates

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=719288 is already been fixed in Utopic/14.10:

linthesia (0.4.3-1) unstable; urgency=medium

  * New upstream version.
  * Bug fix: "Crash when opening any MIDI file", thanks to Pierre Rudloff
    (Closes: #719288).
  * Bug fix: "please provide a desktop and menu file and icons", thanks to
    Markus Koschany (Closes: #737964).

 -- Cleto Martín <email address hidden> Thu, 31 Jul 2014 21:41:54 +0100