Do English words have fixed pronunciation?

Maybe what you are getting at is the fact that Chinese associates tones with different basic meanings (~words), whereas English uses tones to add additional meaning (including emotion) to either individual words or groups of words.

The same English word can often be pronounced with a high, low, rising, falling, or whatever tone. The basic meaning of the word typically remains the same when the tone changes. It is just "colored" differently. Tone can change the occurrence of a given word's connotation slightly or change the meaning of its surrounding words (context), but the basic meaning of the word typically stays the same.

English speakers don't tend to think of those different ways of saying the same word as different pronunciations, however.

Just for fun, try reading this sentence repeatedly, putting stress on a different word each time. The meaning changes fairly radically.

I didn't say you stole her money.

I didn't say you stole her money. (John said that.)

I didn't say you stole her money! (Emphatic denial.)

I didn't say you stole her money. (But maybe I insinuated it)

I didn't say you stole her money. (I said John did that.)

I didn't say you stole her money. (I said you borrowed it.)

I didn't say you stole her money. (I said you stole John's money.)

I didn't say you stole her money. (I said you stole her car.)

No change in the words or their pronunciation. Just a change in stress. Now think about how you would express those different meanings using Chinese.