The sound clip was recorded on April 15, 1885 and was made in his laboratory in Washington, D.C. on a wax-disc. You can hear Alexander Graham Bell say, very slowly:
“Hear my voice, Alexander Graham Bell”.
To hear the 128-year old audio file you can visit the Smithsonian page here: Listen to Alexander Graham Bell’s Voice.
There is also a short video explaining the technology behind the original recording and how the Smithsonian Museum preserves the recordings and gets the sound off of these formats that are no longer playable: Playing the Unplayable Records Video