Dylan Thomas reads 'In My Craft Or Sullen Art' 

I thought it might be the bobbing, swaying motion of the flame of an oil lamp, which in my fancy has the appearance of the movements of a singer on stage.

@@anthonylindley2234 Or perhaps the gentle noise made by coal when it sometimes hisses as it burns? That would account for both the "light" (of the fire") and the "singing" sound of the coal. Not many people today will recall the experience of a coal fire burning in an open grate! Magnificent poem, magnificent words.

The "singing light" is the old hurricane lamp - it makes a very distinctive sound.