How does Shakespeare manipulate and vary the mood throughout Act 2 Scene 3 of Twelfth Night?

In the opening to this scene Shakespeare creates a comic atmosphere by the use of jovial characters; Sir Toby and Sir Andrew. They are carousing in a drunken, noisy celebration and are shortly joined by Feste. However the merry atmosphere is cut short by the introduction of Feste’s song. The song, despite the fact that it is a love song seems to allude to death with the reference to the lover’s journeys end. This could be Shakespeare’s way of reminding the audience that love will not last for ever, creating a rather dismal mood.
However shortly after, Feste delights in another song which provides the audience with subtle hints of what is to come further in the play. The audience later learn that the song is what sparks Maria, Sir Toby and Sir Andrew’s trick on Malvolio, as it urges that they must live in the present as ‘in delay there lies no plenty’ suggesting that other characters in the play will too realize that time is short and the speed of the plot will start to increase when characters decide to confess their love creating a sense of excitement. Youth’s a stuff will not endure. ’
Suggests to the audience that Olivia’s beauty is fast fading and if she wishes to find love she must come out of her mourning state. It also alludes to Viola as her beauty will too start to fade and she will no longer appeal to the Duke but he will never know that she is a woman unless she removes her disguise. Shakespeare returns the scene to its comical nature with entrance of Malvolio. He is traditionally wearing night attire and as an audience we struggle to take his authority seriously while he looks so ridiculous.

Yet the comical moment is cut short with Malvolio’s reminder to Sir Toby that he can be thrown out of the household if he will not separate himself from his misdemeanours. Sir Toby continues to sing of how he can never change his ways and so therefore must leave, however when probed by Feste that he is too cowardly to stand up to Malvolio social ranks are resumed. We see Sir Toby turn from a merry character to one who can be vicious when he verbally attacks Malvolio addressing the fact that he is in fact nothing more than a steward. Again Shakespeare has changed the mood from humorous to quite serious.
This serious mood continues when Malvolio threatens Maria. Yet it is vital note that it is this threat which too encourages Maria to conjure her plan to destroy Malvolio. This is surprising to the audience as she is the one who is most likely to be affected from the outcome of this trickery as Malvolio is her boss. We are left feeling unsettled as to what the will be the outcome of her physic. Nevertheless Maria reminds the audience that Malvolio is a cheat and fraud who is self-obsessed and we feel less unsettled and more willing for them to follow through with their plan.
Ultimately in the scene Shakespeare has varied the mood greatly. Despite the fact that it is a comedy in this scene a serious note runs throughout. It is also important to note that this scene runs in a cycle as we start with Sir Andrew and Sir Toby wanting to get drunk and the ending of the scene also finishes with them wanting to get drunk suggesting that the play may run as a cycle too. He uses songs to hint to the audience of events to come and also address the fact that time is running out.