It's easy for new talents to call around to different recording studios and ask them to do their demos. Most will say yes. The problem is that most studios use the same scripts for any talent who shows up and to be honest, studio engineers are outstanding at making you sound good, but they're not voiceover coaches.
Asking a studio engineer to direct your demo session is like asking the dolly grip on a movie set to sit in and direct the film. You will not get the best direction possible during one of the most important days of your early voiceover career.
It's vital to work with a coach for several months before you even think about recording a demo. I know it's cheaper to just call a recording studio and book your demo. The problem is that your demo will sound good but your reads will sound amateurish and unprepared. In a play, you're not just going to take your script, walk out on the stage and dazzle the audience. It takes weeks of rehearsal time to master your performance. This is how the start of your voiceover career should work as well. Never just walk into a recording studio unprepared and put all your faith in the studio engineer. You will be disappointed.
The difference between rushing to a recording studio to do a demo and taking six months or longer to get coaching upfront is like night and day! If you're brand new to this fabulous business, don't screw it up by doing things in the wrong order. Get the proper training and THEN get your demo recorded.
As for DIY demos, don't go down this dark and dangerous path. Self-produced demos are a recipe for disaster. Great if you want to impress your friends and siblings but horrible if you're going to impress a talent agent or producer. If you’re serious about having a chance to succeed in voice acting, you need coaching upfront and a professional demo produced by an experienced demo producer. Short cuts don't always get you to the right destination. Ask any talent agent in the country and they will tell you the same thing. Having the best chance to succeed in voiceovers requires that steps are taken in the proper order and done correctly.