#1 If you are having problems with a synth/sample/vocal having artifacts at the beginning or end of the sound you have 2 options to treating that.
(1) You can add a “gating” effect that mutes the sound when it reaches a certain dB range, typically the dB setting is lower then the sounds overall dB it is effecting. You can also vary the speed the gate starts, sustains, and stops the muting i.e. You have a vocal that has a breath sound at the end. Add a Gate to the vocal and set the dB range to -16dB. This means ANY and all parts of the sound that is lower then 16dB will be muted. When the last part of the vocal plays the breath will be muted if the breath is -16dB or lower.
(2) Another option is to edit the sample itself by cutting out the artifacts either in an audio editor or directly from your DAW. This can work very well with quick cuts but at times you need a gradual fade of the sound so this option would not work as well as a gate effect.
#2 Compression has 3 uses: Limiters, Gates, & Compressors.
Limiters: are usually used to keep the peaks of a sound down to a certain amount (like 0dB).
Gates: let a certain amount of sound come through if it is at the right volume or higher, once the sound goes below a certain point the gate turns off the sound so no other sound comes through. As written above it is useful to edit sounds with hiss/noise between sounds.
Compressors: have 2 uses. the 1st is to keep the peaks down and add gain to the quietest parts of the sound. This is commonly used while recording sounds as well as post production to make the sound(s) level out and control the peaks/valleys more. The 2nd use (and the most abused) is to change the overall timbre of the sound by adding distortion from over compressing. This can give a sound a great feel, but it is very easy to over do and too many sounds compressed just interfere with one another. If you are using compression, you need to ask yourself WHY you are doing it. Any sound seems like it is better if it is louder so it can be very easy to abuse compression. Try only adding compression to a few sounds and see how much more room you have to work with.
#3 A few ways to “widen” a sound are by using phasers, flangers, chorus, spatial wideners (psycho-acoustic), and reverbs. All of these effects have 1 thing in common, they are all based upon the “delay” effect. I won’t go into each deeply now but I do recommend you research each one to understand them better. The key to using these effects is to know what each effect can do and how to change the settings to fit what your wanting the sound to do spatially and not lose quality or get “phasing” that can ruin a sound/song if it is played through a mono source. With Flangers, phasers, reverb, and chorus you need to really watch the % Wet/Dry mix. With spatial wideners you need to be aware that your not using the same settings for your sounds or they will be placed over one another and become muddy.
The best ways to make sure your sound/song is mono compatible (which is important if you want your song played at a big club or festival were they use mono sound systems) is to use a phasing visualizer like the Waves PAZ plug in. I use this on ALL of my songs to check for phasing problems. Using this takes away most of the anxiety of trying to figure out if the sound is mono compatible. Another thing to do is to check your sound/song is mono compatible by having it play through a mono source. If your mixer does not have a mono button you can always add a stereo-to-mono plug in and turn it on from time to time to check sounds/song.