Does your son attend a public school? If he does, they should offer speech services and should be able to evaluate him. The examples you gave (wif instead of with, and trouble with r's) are normal at that age. It's possible you really have noticed things about his speech that are not normal, but the "th" sound is completely normal through kindergarten, and the r's will usually be corrected in 1st or 2nd grade if they have not developed on their own.
My SIL is a speech therapist in a public school, and I have talked to her many times about both of my boys. My kindergartener is working with a speech therapist on the "th" sound, and my SIL commented that not being about to produce that sound at 5 years old is completely age appropriate. She wishes she had the resources to work with kindergarteners on that sound. Also, my niece received services for the "r" sound when she was in 2nd grade. I believe my SIL would have addressed it sooner, had this niece been her child or a student at her school, but was happy to know the girl was working on the sound.
I wouldn't stress too much about the hearing test. Keep in mind that school routinely screen for hearing, but that it is just that, a screening. Many things can cause the child to fail the screening. It could have been fluid in the ears, he could have been thinking about something else, he might not have been feeling well, maybe there were other things going on in the room. That is the reason they suggested he see an audiologist. Failing the screening test could mean something and it could mean nothing. They are simply suggesting you have it checked just to be safe.
Talk to the school about speech services, and talk to your ped about your concerns. In the meantime, I would just try and relax and listen to what he's telling you. Support and encourage him at school. He will get more comfortable in time.