Lisa Passey and Wayne Dale left son Kian Dale in an upstairs bath seat for at least 13 minutes, Worcester Crown Court was told.
Ms Passey was with a friend downstairs and Mr Dale was "socialising" and using his computer".
Ms Passey, 28, and Mr Dale, 45, both deny gross negligence manslaughter
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Opening the prosecution, Jonas Hankin QC, said: "When, finally, Wayne Dale went upstairs to the bathroom, baby Kian had drowned."
Mr Hankin said that family friend Jeanette Morgan had visited the then-couple's home in Kyreside, Tenbury Wells, Worcestershire, on 26 September 2015.
She and Ms Passey "drank coffee together and smoked cigarettes" in the sun on the patio, before Mr Dale joined them, he said.
Mr Hankin added: "She asked him to burn a copy of the UB40 album, playing in the kitchen, which he did."
After Kian was discovered lifeless in the water, Ms Passey dialled 999 telling the operator her baby had "drowned in the bath", the court heard.
The court heard Mr Dale, of no fixed address, told police officers he "had a beer, rolled a cigarette outside and burned a CD" and that he had only left his son for "a couple of minutes".
A post-mortem examination found the child's death was consistent with drowning, including what was believed to be soap bubbles in his lungs.
Ms Passey, of Tenbury Wells, initially claimed she had not run a bath, but twice changed her account, the prosecutor alleged.
The trial continues.
A couple left their 13-month-old son alone in a bath where he drowned as they entertained a friend, a court heard.