If you use the rules on Sleep in Xanathar's Guide to Everything
From XGtE "Sleep" (p.77):
When a creature sleeps, it is subjected to the unconscious condition.
Normally, a character subjected to the unconscious condition:
is unaware of its surroundings
However, Xanathar's also says (ibid)
A sudden loud noise — such as yelling, thunder, or a ringing bell — also awakens someone that is sleeping naturally.
Whispers don’t disturb sleep, unless a sleeper’s passive Wisdom (Perception) score is 20 or higher and the whispers are within 10 feet of the sleeper. Speech at a normal volume awakens a sleeper if the environment is otherwise silent (no wind, birdsong, crickets, street sounds, or the like) and the sleeper has a passive Wisdom (Perception) score of 15 or higher.
Thus, even though Xanathar's says that sleeping characters are unconscious, it also says (specific over general) that unlike other unconscious creatures, naturally sleeping characters are able to Perceive their surroundings and respond to sounds.
A trancing character, by definition, is not sleeping. Instead, they are meditating deeply. The natural English usage of meditation (and semiconscious) implies that they may be less aware of their surroundings than someone who is fully conscious, but that they are still more aware of their surroundings than someone who is asleep. Thus, a trancing elf should have a better chance to Perceive things than someone who is sleeping.
Advantage on a roll is the standard way to show that the situation is favorable for the person making a check; when no dice are rolled, as in a Passive check, favor is applied as a +5 bonus. A +5 bonus on a Passive check is equivalent to a -5 on the DC of the task and easier to compare.
Thus, for a trancing elf:
A sudden loud noise — such as yelling, thunder, or a ringing bell — will bring them out of their trance.
They will notice whispers if their passive Wisdom (Perception) score is 15 or higher and the whispers are within 10 feet.
Speech at a normal volume will also be noted if the environment is otherwise silent (no wind, birdsong, crickets, street sounds, or the like) and the trancer has a passive Wisdom (Perception) score of 10 or higher.
If you use the core rules only
Without Xanathar's, a normally-sleeping character does not have the unconscious state. Rather, they can be treated in certain situations as unconscious and others not, as the DM is told to apply the condition intuitively.
Thus, to resolve the OP's question, we now have to consider four states:
Conscious / Awake: the default condition with no penalties to Perception
Semiconscious / Trance: Perception to be determined
Lacking Consciousness / Normal Sleep: Perception treated by the DM intuitively
Unconscious: No Perception or awareness of surrounding conditions
If it is not immediately obvious that "semiconscious" should be closer to "conscious" than "lacking consciousness" is, recall that elves don't sleep, and instead they deeply meditate. The natural English usage of these words indicates that meditation is between consciousness and sleep.
Thus, we have two states to adjudicate between "no Perception" and "full Perception". We are told that the lower state should have Perception applied intuitively, and by natural language we know that the higher state will be more perceptive. This is as far as RAW can get us. Personally, I would simply default to Xanathar's rules on sleep for the sleeping state, but in the absence of these DM's are free to intuit as they will.