Technically the moment you are using the work of one person to benefit yourself (or another - but why would you be doing it for them if it wasn’t to benefit you somehow) you need rights assigned and royalties are due.
If I were to walk down the road dressed as Iron Person, singing Thunderstunk, to promo my boss’s Pizza Hutch my boss is using DC’s Avengers franchise assets and AC/DC’s assets to bring value to himself. He should definitely be paying DC & AC/DC for the leg-up this gives him.
From what you say this is indeed a business you are running and the voiceover work is part of your sales infrastructure. Therefore the performer definitely has a right to ask for one or all of:
- a lump sum,
- ongoing royalties
- visible credit (his name on the video)
- to show in his portfolio
In real terms though it comes down to individual agreements. In that situation I would look at your YT Channel and decide if you were like:
- Coke or Nike in which case it is all stops out, or
- Nowhere Neville in which case I would probably take the initial fee and show the work in my Portfolio if the show looked ok. Maybe require a visible credit if I thought the show might bring me more business (but I also know that most times small players agree and then never do that so it is bit pointless - and they never succeed with such an attitude).