Why is it, when you install a release of a distro, you are "stuck" to a certain version of all software? Why is there no easy way to install or upgrade to the latest version of a particular program? For example, I have some issues with SoundConverter, which on my system is version 1.5.4, the latest version being 2.1.2. It would be nice if I could just enter a single line in terminal or do a few clicks and get the latest version.
I'm using Linux Mint Maya, in this specific case.

Because most distros start testing with package version 'X', and keep it at 'X' while normally backporting any security fixes from newer versions. That way you have software that is tested to work with your distro.

You can find plenty of distros that do allow updating software to newer versions. However, that is more common with 'rolling release' distros which do have a habit of breaking.

A distro comes with a set of stable programs that are tested by the development team. This team also took care of possible dependencies that the programs might have.

In most cases you aren't "stuck", you might be able to install a newer version of a package. If this can be done easily depends on the developers of that particular package. Some provide the package in a form that can be easily installed using the package manager that comes with the distro (apt/rpm/yum etc). In other cases there is only a source package available (which needs to be build by the user). The latter isn't easy for the average user.

I've just had a look at the SoundConverter web page and they only provide a source package.

EDIT: I'm slow this morning......

I downloaded the deb package for SoundConverter from packages.debian.org, which was version 2.0.1. Not sure if it's a good idea to do it this way, but worked this time.

Installing packages from outside the repos configured for your running system might break the dependency system and mess stuff up badly. It can be done safely, but after making sure its safe and you wont change some package needed by many others.

And a distro-independent method - compile from source and install it in to the default /local prefix.

how true

you "can" install anything
just sometimes NOT easily

a good example is the DEVELOPMENT version of The Gimp
even on fedora the DEV version often NEEDS !!! to have installed a NEWER version of glib,glibc,packageconfig,gtk,....
now some of these are a PAIN IN THE "bleep" to have two versions of the software installed

so you CAN but it might not be easy

also you REALLY do ( most of the time) want a distro that is STABLE

RHEL6 is very stable
BUT at a cost

The versions of used programs are a bit OLD
Gnome2
Gimp2.4
libpng12
and the "2.6.32 " version of the KERNEL