Midi files are often constructed using a channel concept, such that each part can be placed on a different channel. The author of the file has control therefore over volume, and stereo balance (among other things). Midi players such as the Roland program use the channel numbers to single out a particular part.
The files on this site are normally constructed as follows:
|Chan 8||Soprano||Stereo balance full left||63% full volume|
|Chan 7||Alto||Stereo balance full left||63% full volume|
|Chan 6||Tenor||Stereo balance full left||63% full volume|
|Chan 5||Bass||Stereo balance full left||63% full volume|
|Chan 4||Whichever part is enhanced||Stereo balance full Right||78% full volume|
|Chan 3||Solos or other parts||Stereo balance centre||50% full volume|
|Chan 2||Right hand of accompaniment||Stereo balance centre||50% full volume|
|Chan 1||Left hand of accompaniment||Stereo balance centre||50% full volume|
Thus for instance, a soprano 2 file will be as above, except instead of both soprano parts being on channel 8, only Soprano1 will be on channel 8, and soprano 2 will be on channel 4. This means that soprano 2 part will be 15% louder, and balanced to the right, whereas other voice parts are balanced to the left. Soprano 2 part should therefore be audible above the rest.
Also, because the Roland program uses channel 4 as a "guide" channel for its original karaoke function, the singer has extra control over the sound insofaras the sound can be further enhanced, or even muted - very helpful in finding out if you can really hold your part against all the rest when your colleagues get it wrong!
Possibly the main disadvantage of the above arrangements is that any performance dynamics which have been placed in these files will have been removed, and the volumes settings for any particular part will be constant for the duration of the piece. This makes for a drab performance, but for learning notes and becoming familiar with the feel of singing your part within the whole harmony I think it is a small price to pay.
As far as possible I always construct the midi files to have the correct bar numbers to match the scores, even if I have truncated the files to avoid long sections of instrumental accompaniment, or if extra rhythm-setting notes inserted at the front. This means that when practicing with a program such as Roland (where there is a running display of the bar number), one knows exactly where one is in the score.
If anyone wishes to pursue further the methods used to create or modify midi files, there are many music sequencers both on the market, and indeed even one or two freeware versions. I use Cubasis, which is a simpler version of Cubase. Other names are Cakewalk and Sibelius.
There also many sources of midi files on the Internet. The Choral Public Domain Library is one which I have used often for midi files as well as scores.