Generally, your sound levels should stay at around -2db to -3db, you don't want to blow your mini speakers or anyone else's by having the wrong volume levels. Another thing to check for is once you are sent an audition or are submitting for an audition make sure you have the correct bitrate settings when you save the end file could be 128 bitrate but can be other than this. This information is usually given to you at the first instance.
Of course, the main thing you need to do is be careful to check exactly what it is that the person requiring the voice over requires, maybe they have certain requirements that in the speediness of sending sometimes get overlooked by the voice over artist and they may want certain emotions put in to the mix. Always read everything carefully to ensure that you will be spot on with your audition. Hopefully then, you will be the VO who is picked for the job.