In Act 1 of Hamlet, what is the effect of having the sounds of the carrousing of Claudius reach the watchers on the platform?
You are of course refering to the sounds of the partying that reach Hamlet and Horatio in Act I scene 4. Here, there is a basic juxtaposition between the serious purpose of Horatio and Claudius and the frivolous partying of the new King, which shows his irresponsible nature. Consider Hamlet's assessment of this tradition:
This heavy-headed revel east and west
Makes us traduc'd, and tax'd of other nations:
They clepe us drunkards, and with swinish phrase
Soil our addition...
Such a "tradition" therefore only serves to highlight the changes that have occurred in Denmark. Claudius is not doing a very good job of being King, and the way that he is presiding over an opportunity for drunkenness and vice whilst Denmark is facing a military threat in the form of Fortinbrass and Norway is certainly at best questionable and at worst reprehensible. For Hamlet, the rightful heir of Denmark, to hear such sounds whilst he is waiting to see the ghost of his father only serves to underline the way in which there is "something rotten in the state of Denmark" that has occurred with the change of leadership.