What is Public Domain
No one can claim ownership of a song in the public domain, therefore public domain songs may be used by anyone. Music recordings, however, are copyright protected separately from musical compositions.
NOTE: There are NO, sound recordings in the Public Domain in the USA.
If you need a music recording - even a recording of a public domain song - you will either have to record it yourself or license a Royalty Free Music recording.
To prove public domain status in the USA, you MUST find a published copy of the song with a copyright date of 1922 or earlier. Most PD works have arrangements under copyright protection, and new arrangements must be derived from a PD source. PD status is determined by the laws of the country where the music is used. ( from the PD Info site )