It shouldn't take more than a few lines of code and $10 worth of memory to change the box: my phone can record 200 hrs of speech!
Your phone may be good, but has been stated many times before, your phone is not certified to withstand the G's or the temperatures and pressures that a CVR is expected to withstand.
Agree that $10 and a phone is not a viable option.
SSD using NAND flash is being used in CVR's and which are many times more resistant to shock (G's), can be fully encapsulated (potted) to withstand great temperatures, be waterproof are not sensitive to pressure.
Besides, if worries, use the same enclosures as is currently used.
The only question is how much memory to stuff in the recorder, but I will venture that $1000 will give you many thousands of hours.
If the recorders of MH370 are found and it proves that only the last two hours of (possible) silence is available, when the first two hours of flight may be the most critical, then I hope the industry and regulatory agencies will recognize that improvements are in order.