Voiceover demos are your calling card and I can't stress enough how important it is to get them professionally produced by a demo producer who knows the industry well. It will be tempting to take shortcuts and do the demo yourself but mark my words, a talent agent or producer will be able to sniff a DIY your demo in five seconds flat.
When I first started in voiceovers, I attempted to do a demo myself. Unfortunately, I nearly got blacklisted by two high-profile talent agencies for wasting their time and not taking this seriously. Sure, you may get away with a DIY demo on a Fiverr profile but if you want the best chance at having a career in voiceovers, get plenty of coaching upfront and have your demos professionally produced. Talent agencies and high-level production companies will tell you the same thing.