Tom saved a message using the least significant bit (LSB) method in a sound file...

Comprehensive and Detailed Explanation From Exact Extract:

In steganography, the carrier is the file or medium used to hide the secret message. In this example, the sound file is the carrier because it contains the hidden message embedded using the least significant bit method. The message is the payload, and the website is merely the distribution platform.

LSB is the embedding technique, not the carrier.

The message is the payload, not the carrier.

The website is not involved in data hiding.

NIST and steganography references clearly define the carrier as the container holding the hidden data.