According to Channel 4, there are three main reasons why there are sound dips in the live feed:
1.
Strong Language and Adult Discussions - as you probably know, channels are prohibited from broadcasting any strong language or adult material before the watershed begins at 9.00pm, and ends at 5.30am.
2.
Privacy of people outside the Big Brother house - housemates often discuss things about people on the outside world. The people that they are talking about have not agreed to take part in Big Brother, and therefore they most likely won't want any information about them becoming public knowledge, so such discussions are removed.
3.
Legal Matters - if a housemate says something that is potentially libellous, it will be cut to avoid any legal issues.
However, it is often claimed (but never confirmed) that they will dip the sound and/or picture if something takes place that they'd rather keep for the highlights show.
Also, in very extreme cases, they will dip the sound and/or picture if there is an argument going on in the house (an example of this was Spitgate in BB9.)
More information can be found here:
http://www.channel4.com/bigbrotherlive/