Your best bet is to spend $20 and go to your nearest retail center. Low quality PC speakers are dirt cheap and will sound 100 times better than a mobile phone's speaker. Either that or get a pair of $2 headphones.
as the title: Is it possible to use a Mobile phone as a PC speaker? if yes.. how?
yes its possible. install software warlex mobiola headset in you pc and your mobile so you can use your mobile as a headset via bluetooth wifi or cable.but it depends on what mobile you have , it works on iphones, android phones and symbian(nokia) phones.i have tried it on nokia n82 and it worked perfectly.