This is a very broad, indeed all-encompassing question. I suspect that the intended emphasis is not so much on whether all political parties in a representative democracy are treated fairly by the mass media, but rather on the conflicting views held by sociologists on the role played by the media in a modern liberal democracy. The following points are therefore based on this assumption.
The press and TV are generally seen as the most influential media. The key difference between them is that the press has considerable freedom to print what it wants, while TV is bound by law to be impartial, with clear guidelines covering content. In the UK four firms produce 85% of national daily and Sunday newspapers. This trend is being reinforced by mergers; media are also increasingly owned globally (which may narrow the range of political views on offer) - the Murdoch example is the classic case in point.
The various theoretical approaches should be discussed:
Pluralist theory argues that the mass media simply delivers what readers and viewers want. It may exhibit bias, but its main influence is to reflect and reinforce existing attitudes and beliefs. Given there are a diversity of interests and beliefs, this is reflected in a wide choice of papers and an ever increasing choice of TV channels, many of them highly specialist, and available on SKY. Hence no single group or elite dominates society, and so media owners only dominate their own areas; they are all competiting with each other for business. A wide range of views is presented by the media, allowing consumers to make their own choices, including political choices at elections.
Marxist theory stresses the power of the media (and thus media owners) to control, rather than simply influence, people. The media transmits a conservative, conformist view, and promotes elite attitudes and values. It works against change and diversity, and thus against the interests of the majority, supporting the interests of the rich and the powerful white, male, middle class (who include owners) who have a vested interest in portraying capitalist society in a positive light. It thus promotes a 'false consciousness', preventing people from seeing the reality of their situation, and persuading them to accept things as they are. It is not simply political content and reporting that support the narrow viewpoint of the capitalist system, but also the content of entertainment programmes and coverage. To quote Miliband, media owners are overwhelmingly people 'whose ideological dispositions run from soundly conservative to utterly reactionary'. Hence, on this argument. the media will be ill-disposed towards political change, the rights of minorities and governments of a left-wing disposition.
Post-modernists stress the importance of style and images. In a media-saturated society style itself has become a commodity, and is more important than content. So the mass media increasingly dominates our definition and sense of reality. Our world increasingly consists of images from TV broadcasts, adverts, computer games, videos, designer labels, etc. So there are 'multiple realities' - certainty and fixed standards have been replaced by uncertainty, scepticism and fantasy. We see and read what we want to see and read - so media images encourage superficiality rather than substance, and the thirst for constant change. So successful media owners are those who recognise this and react accordingly - seen, for example, in the 'dumbing down' of papers such as The Times. Politically the focus is likely to be upon personalities and controversies rather than on detailed and dispassionate examination of policies and evaluation of parties.
On the basis of the above frameworks, the following reflections are worth considering:
The press, almost without exception, does present a political view, and most papers offer advice on which party to support in an election. At one time, the press had a very heavy Conservative bias, but this was certainly not so in 1997, 2001 and 2005 when most papers supported Labour - including the SUN, and despite Murdoch's known Conservative affiliation. Moreover, the political content of newspapers is largely determined by editors rather than owners, though of course it is owners who appoint editors.
The press is, however, now becoming increasingly critical of the Labour government in general, and of Brown in particular. Conservative fortunes have greatly improved since Cameron became leader, and several papers that supported Labour in the last three elections might well switch to the Conservatives next time. The Liberal Democrats find it hard to attract coverage or media support, though the party tends to receive fairly favourable treatment from both The Guardian and The Independent. In any event there is very little evidence that press support for parties has much influence on readers' voting - people tend to buy papers that support the political views they already have. And, as pluralists would point out, choice of paper is seldom dictated by political content; most readers are more interested in the sport, entertainment, etc.
Nevertheless press editors are certainly powerful opinion leaders since they determine what goes in the paper and how it should be covered. In other words, they are effectively agenda-setters. Tabloids in particular are very heavily influenced in their content by what their readers want - hence the stress on salacious stories, soap operas, media personalities and celebrities, moral panics, etc. Because the media is strongly involved in setting the political and social agenda, this is why governments try so hard to influence it through 'spin'.
Although in principle broadcasting must be politically neutral, again its choice of how much stress to give particular items of news, and how to present it, is critical - so we see the stress on news that is immediate/sensational/highly visual/readily grasped. This would tend to support the post-modernist view.
I hope this is helpful.