A to D conversion

I would like a technical answer as to the cause of a "lisp" on some digitally processed speech.  Speech sounds like a mis-tuned AM radio. Specifically I am referring to the commercials on internet radio on radio station KRLA 870. I am mostly going on iheart radio using an i6. The program content is 99% good but when the commercials come on the lisping starts.  It is very annoying and seems to occur only on KRLA, other internet radio stations do not have the problem. FWIW the over the air audio does not have a problem.
I am amazed that put up with it but I would like to know what causes this problem..