All sound producers are likely to come across a situation when it is necessary to lay vocals to an already pre-mixed instrumental. For hip-hop it is fairly a common case, that’s why there is a set of specific skills that allow throwing vocals into the mix. A pre-mixed instrumental is as a rule compressed (and sometimes very highly compressed) and maximized for attaining a certain volume level. The problem is that loudness here means not only turning on the volume level. After a certain point loudness can be raised only by increasing density of the mix but not that of the volume level. This comes down to the situation when we have a too dense backing track, there is simply no room for vocals left. By the way the first thing that is necessary to do is to considerably (2db-4db) reduce the volume level of the track in order to force everything work properly. As a result the louder the instrumental the quieter the songg after mixing mastering audio services. It is ironical, isn’t it?
First of all make vocals sound maximum well by their own, and don’t forget to check how they sound when combined together with the beat.
When clearing the space for vocals on the backing track try to use an equalizer as little as possible. Here is more the matter of dynamics. Make the instrumental lower and use an expander or transient designer to process peaks of a beat. From three to six decibel of a dynamic range are as rule enough for a start.
Sometimes an instrumental is so highly compressed that there are no transients for a transient designer left to work with. In such a case you will need to be inventive. I often choose drums which mostly match or compliment the beats in a mix and program them over track.
You can find some more space if use M-S processing: it allows to separate mono- and stereo-elements of a signal. Lowing the centre point (M) and raising stereo (S) will allow you to create an additional room for vocals. For example, you can use Waves Center for this task or an equalizer with mid-side processing.
If there is something in the backing track that really interferes with the vocals cut it with an equalizer. Try to do it carefully and reasonably. It is impossible to exclude compromises when do equalizing of the initial instrumental.
To learn more please check How to Mix Vocals to a Pre-Mixed Instrumental better.