What is it?

Riddles are short poems or texts that ask a question that seems difficult to answer. In her famous riddle below, Catherine Fanshawe is talking about something—but what is it? And why is this riddle in the EnglishClub pronunciation section? (You can find the answer here.)
Special note - this riddle uses the following contractions:
'twas = it was
'tis = it is
'twill = it will
o'er = over
e'en = even

'Twas whispered in Heaven,
'Twas muttered in Hell,
And echo caught faintly
The sound as it fell;
On the confines of Earth,
'Twas permitted to rest,
And the depth of the ocean
Its presence confessed;
'Twill be found in the sphere
When 'tis riven asunder,
Be seen in the lightning
And heard in the thunder.
'Twas allotted to man
With his earliest breath,
Attends him at birth
And awaits him at death,
Presides o'er his happiness,
Honour and health,
Is the prop of his house
And the end of his wealth.
In the heaps of the miser,
'Tis hoarded with care,
But is sure to be lost
By the prodigal heir;
It begins every hope,
Every wish it must bound,
It prays with the hermit,
With monarchs is crowned;
Without it the soldier,
The sailor may roam,
But woe to the wretch
Who expels it from home!
In the whisper of conscience
'Tis sure to be found,
Nor e'en in the whirlwind
Of passion is drowned;
'Twill soften the heart,
But though deaf to the ear,
It will make it acutely
And instantly hear.
But in short, let it rest
Like a delicate flower,
Oh, breathe on it softly,
It dies in an hour!