These days, with aliexpress, fleabay, and many other places that sell mass produced products over a very wide range of quality and price, I suspect it would be more expensive to develop your own. Unless something has very specific requirements, functions, reliability you'll be better off buying up products in bulk.
With that said, just for voice recording, there are plenty of 'sound recorders' using micro's floating about on the web. The simplest I ever made recorded in 12-bit quality at 16kHz and just used an AVR, a flash chip, and a transistor driving a speaker. The cost of those parts, plus the PCB and the most difficult part - the housing, so its user friendly - would probably be more than purchasing a much 'better' product.
For one-off hobby, or making 'toys' to play with, designing something yourself is great, but for industry, there is a reason why so many chinese factories advertise for custom manufacture - you can decide the level of quality, and the cost is often very low for high volume parts.