To date I've tried the Transcend DrivePro 200, the BlackVue DR500, the Mini 0801 and the Mobius Action cam, most of which gave really good results in both video and audio but one thing I have learned is that you DON'T get what you pay for. By that I mean that the most expensive, The BlackVue at around £200 with built-in WiFI, GPS logger and it's own app, was not the best overall dashcam for my needs. It had a really wide 'fish eye' type lens and the sound was out of sync with the video no matter what setting I used it on. Without going into a lot of detail I settled on the Mobius Action Cam which costs around £56 depending on lens choice and mount type, BUT it gives excellent day and night video recordings and the sound is equally as good. I have also used the camera pointing into the car and found that it gives a great way to improve lessons and help clients to build their driving skills.
If you think you might be interested in one of these camera's then check out 'Techmoan' who has a full range of reviews on this stuff on YouTube and to purchase the Mobius Action Cam head along to Joovuu which is where I bought two of mine. They are quick on delivery and keenly priced.
Just as a footnote, some insurance companies offer a discount if you have one of these dashcams fitted, so it might even save you money and a lot of arguments in the event of an accident claim.