Forearms are just as accurate just like thighs are just as accurate, size of BP cuff is what is necessary to ensure an accurate measurement of BP, I've worked for a cardiology clinic for a few months and the docs have said, on obeese patients, with forearms like normal sized patients upper arms, is no different. You wouldn't use a thigh cuff on 90 lb grandma's upper arm and expect her BP to be very much. The reason your sounds are faint is that when you put your steth bell over the radial artery, the bell usually hangs off or isn't directly over the pulse point. If you have alot of obeese patients from this practice and are going to take BP's on wrists alot, it might be a good investment to pick up a ped's steth, the bells are smaller and can usually accomodate a radial artery site, and your sounds will be much better, however, you should cut down the tubing and make them as short as practically possible. The further sound has to travel, the more the tubing will absorb, the less you will hear. I had a pediatric/NICU RN as a clinical instructor who had two peds steths, one was a cut down tubing, and the other the long long tubing, and sounds are better heard with a cut down model. Just go with a cheapie, no need to spend 200 dollars on a master cardiology for BP's.
There's always a palp BP as well if nothing else works, you'll get the systolic, but no diastolic pressure. At the very least, you'll have the information you need to administer your BP med.