Sometimes the clients ask for a “sample” voiceover in my gigs and they send me a small script for me to read promissing a long gig if they are satisfied. Obviously, there is a risk that this small script is the gig itself and once I record it, I will never hear from them again. So what I do is inserting in the background a random 1000HZ sound at 30% of the volume. A beep about 2 seconds long that randomly appears and disappears mixed with my voice.
I’d advice if you work in the voiceover category to download a 1000Hz sound (you can easily find it on Youtube), lower it down to 30% and save it for future use.