Here are a few Midi files I've created. You'd never guess that I'm a Beatlemaniac, would you?
MIDI is a very generic format for describing music. Actually the original midi format does not even have any instruments defined in it. Only channels and program numbers. Professionals would connect different channels to different midi playback devices and select the instrument patch that would be controlled by the program number.
To make things simpler for the rest of us, some general instruments were defined in the general midi format and playback device manufacturers could choose to pre-select instrument patches for these program numbers. All of this can be bypassed with configuration settings, but for the average user this meant that a midi file would play instruments that sounded somewhat like what the creator had intended.
In terms of the average computer sound card, the card or the driver contains the instrument patches that are played. Depending on how good the card is this instruments might sound slightly similar to the intended instruments, or the sound might be very good. This is purely dependant on how much memory is allowed for the instrument patches and the quality of hte patches themselves. If you have a rather cheap card, the instruments probably sound pretty bad and you don't think much of midi music.
Rather than relying on the quality of the patches supplied with your card, there are also ways to change the patches used for playback. There are various ways to do this, but the easiest for me, on Linux, is to use Timidity++. Rather that using the instruments provided on the card, Timidity++ has it's own patches. It reads the midi file, assembles te music from it's own patches and plays the results as plain sound through the sound card. The bottom line is, reguardless of how cheap the sound card, the midi file is played with high quality instrument patches. Timidity++ can also be configured to use other patches, even commercial ones available from Roland and others. This allows you to tailor the music playback to use the instruments you want. Even the patches that come with Timidity++ sound much better than most sound card instruments.
If all this is news to you, then I suggest investigating how to re-configure the instrument patches for your sound card or give Timidity++ a try. There is a Windows version, but, being UNIX-based, it's difficult to configure on a Windows box. Either way, it's still cheaper than buying the latest and greatest sound card just to get decent midi output.
Most of these midi files were created with noteedit from music in a book titled The Beatles Complete scores published by Hal-Leonard
Of course the playback quality will depend on your sound card or midi software. Also, it is very difficult to capture the little nuances of music performance. This is why most computer-generated music sounds stilted and lifeless. Computer timing is much more precise that musicians.
Then you have to deal with the quality of the instrument patches used. Drums are rather generic and simple. these will generally sound O.K. Brass instruments are generally not too bad, unless you have a very cheap card. String instruments take quite a bit of memory, so will not sound the greatest unless you have a very good card, or have very good patches. Then we get to Guitars! The general midi format defines 8 different guitars AND 8 different Bass guitars! And this is rarely enough. First, Acoustic guitars. These can be stringed with nylon, cat gut or steel strings. Then there is how it was amplified? A microphone in front of the guitar sounds different from an internal acoustic pickup. And if steel strings are used, and electrical pickup can be placed over the sound hole. Electric guitars have their own problems. Most have three different pickups, which can be selected in a variety of combinations for different sounds. Then there are various distortion boxes and pedals commonly used as well. The bottom line is, you can never quite get the right sound for a guitar in a midi file unless you select custom patches, and these will not play back right on other machines unless the same custom patches are used.
Having said all that, I've tried my best to select standard patches that sound close to the music. This is also why I've started with songs that had quite a few other instruments rather than the regular guitar-only songs. I also dispise midi file that try to include the vocals, since midi can not sing. Therefore I've left the vocals lines out of these songs.