In Indo-European languages, changing the pitch of the voice usually does not change the meaning of a spoken word or sentence. We illustrate this here, using as our speech sample one of the finer samples of political rhetoric of the early 3rd millenium. (No, not Obama, Bush. Dubya asserting that peace with fish is possible, which is of course enormously reassuring.) In addition to the original speech sample, we add two further samples which have been processed with Hideki Hawakara's "Straight" software, which can decompose speech into its pitch and formant contours, and resynthesize it after the pitch contour has been altered. The result sounds remarkably realistic. So here, then, is Bush, pleading for peace with fish, first normal, then with a steadily rising, and then with a falling pitch contour. You will note that, after the pitch manipulation, the speech remains comprehensible (at least, it is no less comprehensible than it was on the outset).
(Note, however, that, while pitch does not carry much semantic information, it does contribute to 'prosody' (i.e. allows speakers to put emphasis) and it may make it easier to follow the speech of any one speaker in a crowd through pitch tracking. Also, in some languages, such as in Mandarin Chinese, changing pitch contours can change the meaning of a word.)
Bush pleads peace with fish
Bush pleads peace with fish, rising pitch
Bush pleads peace with fish, falling pitch