That's nearly impossible because of the lack of calibration of your test platform (your iPhone + headphones + the way you set it up). You need an audiologist type of set up if you're interested in measuring volume.
Measuring pitch the calibration is much less of an issue.
No problem. Note that if you are looking into this in the context of sound therapy, you should know that what matters most is the pitch, not the volume. Targeted sound therapy takes the pitch as input and crafts a sound based on your pitch. I've never seen any sound therapy that uses the volume as input.