Final answer:
The thyroarytenoid muscle is responsible for tensing (elongating) the vocal cords to change the pitch of the voice. Other muscles like the omohyoid, sternohyoid, and thyrohyoid also affect the voice by managing the hyoid bone and larynx position, but do not directly tense the vocal cords.
Step-by-step explanation:
The only muscle that tenses (elongates) the vocal cords is the thyroarytenoid muscle. This muscle is one of the muscles within the larynx that adjusts the tension of the vocal cords to change pitch. When the thyroarytenoid muscle contracts, it shortens and relaxes the vocal cords, making them less tense and thereby lowering the pitch. Interestingly, higher pitches are produced when the vocal cords are stretched and tensed, which involves different muscles such as the cricothyroid muscle.
To control the position of the larynx and thus affect voice tone, other muscles also play roles; for instance, the strap-like infrahyoid muscles depress the hyoid bone. Specifically, the omohyoid, sternohyoid, and thyrohyoid muscles work together to manage the hyoid bone and larynx position. However, among these, it is not the thyrohyoid but the thyroarytenoid that has the direct role of affecting the tension of the vocal cords.
The sternocleidomastoid muscle moves the head but does not affect the vocal cords. Similarly, the orbicularis oris muscle controls the movements of the mouth and has no role in tensing the vocal cords.