The Beatles' Yellow Submarine contains a cut-up of a Sousa march that apparently came from another record (as described in http://www.beatlesebooks.com/yellow-submarine). Was this the first instance of a released record sampling another released record?
It looks like sampling predates the Beatles somewhat.
In 1924, Italian composer Ottorino Respighi premiered his symphonic poem, Pini di Roma (Pines of Rome). In this piece, many scenes from the natural and civilized world are portrayed with symphonic instruments, such as an organ and various brass instruments playing low pitched notes to represent the catacombs. The third scene, I pini del Gianicolo, features the song of a nightingale. Rather than conjuring images of a nightingale with a flute, Respighi decided to just cut to the chase and play a recording of a nightingale.
Although neither Respighi nor his audience could have any idea this performance had broken ground on a technique that would transform music in the 20th Century, this was probably the birth of sampling. He took a portion of a previous recording and incorporated it into a new performance.
Most other sources seem to give Musique concrète the credit for sampling starting in the 1940's.
As far as sampling from a released record is concerned, the first example isn't valid (as it was a recording not a release ) but the second points towards the setup using record players:
In 1948, a typical radio studio consisted of a series of shellac record players, a shellac record recorder, a mixing desk, with rotating potentiometers, a mechanical reverberation, filters, and microphones.
So maybe we can assume that 'released' recordings were used.