You could start by searching these forums. Here's what you need:
http://eab.abime.net/showthread.php?t=66065
Are you sure resampling them with the sampler yields better results? Because you might be confused about how "good" it used to sound. Getting hiss is unavoidable with certain frequencies, and specially with stuff that isn't loud enough. I suppose you are having a loudness problem when you convert your samples.