Pronunciation of ‘d’ in ‘play’d’
In Byron's day poetry still permitted the archaic pronunciation of the -ed suffix on verbs and participles whose base form ended in an open syllable as a distinct unstressed syllable when this suited the meter.
Spelling the -ed suffix as ’d indicated that the ending was not to be pronounced this way, but as it is in ordinary speech: a consonant closing the open syllable.
In your example, play’d is to be pronounced /pleɪd/, as a monosyllable; Byron excludes the alternative pronunciation with a second syllable /pleɪ.ᵻd/
When Lord Byron wrote, some of the past tense -ed endings that we pronounce as /d/ were pronounced as /ɪd/. So you should pronounce play'd exactly the same way as people pronounce it today, to rhyme with shade. (This was probably in fact the way everybody pronounced played then, in fact. Shakespeare, two centuries earlier, pronounced played with one syllable. Byron was probably being overly careful.)
But if you want to use the same pronunciation as Lord Byron intended, you should probably pronounce clenched with two syllables in the line
Their chins upon their clenched hands, and smil'd.
Shakespeare seems to have consistently pronounced past tenses ending with -nched with two syllables. I don't know if this pronunciation was archaic when Byron wrote.
You read the words as if they were spelled without the elision but you omit the sound that is elided.
So in the example you have provided play'd you would pronounce the 'd' as in the d at the end of housed, played, regained etc...
As explained on (thoughtco.com)
"Elision of sounds can . . . be seen clearly in contracted forms like ... they'd (they had, they should, or they would), haven't (have not) and so on. We see from these examples that vowels or/and consonants can be elided."
Perhaps the easiest way to understand how to pronounce these words is to actually listen to them being pronounced.
Here is the poem being read aloud by Nick Gisburne
I've listed the first three time indexes for words with elision that end in 'd:
extinguish'd - 00:18
chill'd - 00:47
consum'd - 01:06