maryrrf's taping idea was a great one. My current wife did a similar sort of thing for her kids when they were young. Now they cherish the tapes for the sound of their (then) young mother's voice.
I used to sing to my kids when they were younger..doing a variation on 'Hobo's Lullabye':
Go to sleep you weary baby/Let your troubles pass you by/ Can't you hear your daddy singing?/That's a baby's lullabye
Of course they were younger than 18 mo, but anything's worth a try for the sanity of the parent. And little variations on songs can provide a bit of diversion for a weary parent.
Been there. My heart goes out to him! It has the potential to form a fine bond there! That's worth keeping in mind.
Lynn