What is the major cause of the muffled noise from a radio station?

The major cause is "lack of high frequencies in a sound wave".

Explanation:

Muffling derives from either the absence of such a radio signal of a higher or specific frequency. This very same phenomenon has been observed whenever you overhear conversations through some kind of wall and perhaps door. The approach is equalization. This method helps them to raise those frequencies although these overprotective wavelengths decrease.

So that the above would be the correct solution.