There are no absolutes in voiceover, or in acting as a profession. But there are most certainly a number of elements you should consider if you're considering pursuing voiceover as a profession. Training is a given, and understanding who you've created your demos to service is typically overlooked by both seasoned and novice talent alike. Yet it's the endgame and should be your focus from the very start.
Many well-meaning coaches and enthusiasts will tell you that in order to get started in voiceover you should concentrate your efforts on audiobook. However, considering no less than three-quarters of voicing the endless supply of books Amazon (aka Audible, aka ACX) want voiced requires more time devoted to production and editing than actually performing, I'd only recommend you start here if you intend to become a recording engineer, rather than a voiceover. I wouldn't recommend you begin with audiobook if you're just starting out. You may find you'll only frustrate yourself and might never want to hear another voiceover again as long as you live, especially after spending 70-some odd hours editing a book you didn't like in the first place! Audiobook better be a passion of yours if you head down this path, but I wouldn't recommend you attempt to establish your voiceover career here.
The truth is, nearly every client in the States, not just talent agents, requires a Commercial demo first and foremost, even if your focus is Corporate Narration (aka Narrative, Industrial, Corporate Announce, or Non-Broadcast demo.) I can't speak for the UK, South America, Europe, or anywhere else, but consistently this has been the case since we've been surveying this point for more than 25 years. While assets and skills vary from talent to talent, we generally suggest most budding voice talent lead with a Commercial track, per our experience, and again from our continued survey of the industry.
Commercial is king, pure and simple. Sometimes having a Commercial demo as well will support your Narrative pursuits by presenting both tracks in tandem, and will likely afford you the greatest return on your promotional investment as a voice talent. It implies a body of work and demonstrates your ability to be a reliable vocal brand that can go the distance on a national scale. Every production client imagines their product or service rising to such heights, even though currently they may have a small business budget. Their marketing and promotional goals are to expand their business on a grand scale. Otherwise, why promote at all?
Granted, commercial work may not have been your path to date, but it is the standard in voiceover, so you should expect to hear continued requests for this type of demo, even if you're primarily pursuing Industrial work. Again, clients and agents alike are interested in a body of work, just not everything you've ever recorded.
A voiceover demo, by definition, is a professional demonstration of what you do best and what sort of work you want to land most. Rather than a collection of poorly produced, outdated regional spots included simply to take up space, and because you got paid to voice them. No one's interested in hearing a collection of half-baked auditions, either, that took you 2 to 4 takes to record, and ultimately read as hap-hazard and thrown together. You may have included them on your Commercial track because 'it's better than nothing.' It's not. It says you're lazy and reckless, and that doesn't instill confidence. Your demos define you as a brand, and a professional. Or they don't. These are a few of the common miss-steps talent make while attempting to shortcut their way to becoming a professional.
The standards set by Commercial producers more than 40 years ago set the same high-water mark these professionals set for themselves. Today every one in the entertainment field, not just voiceover, is expected to offer and maintain a professional demo of some kind. Demos, by design, are meant to mutually service the specific target audience and the individual talent alike. The mission is to elicit work by defining your aesthetic as a professional.
The fact remains we create voiceover demos to service producers, above and beyond any other target audience as voice talent. And, according to Pew Research, better than 80% of all producers in America establish their careers in advertising and commercial work, where they learn budget, timeline, teamwork (hopefully), and what they need and want from demos.
In any case, you're always expected to offer your best, whether it's an audition, a performance during a session, or a demo. Considering commercial work offers the greatest rate of return and quickest, it's generally the area of the voice industry you're most encouraged to master first.