Apply some noise reduction to reduce background noise from nearby air conditioners or other disturbances. Isolate a one- to two-second clip of dead air and save it as your noise file. Then apply it to the entirety of the vocals.
Equalize the vocals using your program's graphic equalizer. Listen to where the vocals are lacking (low end, mid or highs) and accordingly accommodate for it by increasing the appropriate channel.
Add a reverb or echo effect to the vocals to give them a fuller sound. Add just enough so the echo is an afterthought and not a dominant part of the mix. When the music plays atop the vocals you should hear the echo slightly, if at all.
Normalize or apply a hard limiter to increase the vocal threshold.
Compress the vocals with a ratio of 2:1. This means for every 2 decibels over the threshold it will only produce a 1-decibel increase. Set an "Attack" time between 1 and 20 milliseconds. The attack time determines how quickly the compression kicks in.