Yes.
Unless done with permission of the copyright owner of the original piece of music this would be an infringement.
...of course the act of making an adaptation (remix) of the song is itself a restricted act. So unless it is for personal study (i.e. NOT made available to others), you should be prepared for the copyright owner to take legal action which may include a claim for financial damages.
A recent post at :
http://www.copyrightaid.co.uk/forum/topic275.htm may also be relevant to you.
As for the forum - it is hardly reasonable to blame the owner of the forum for items that are beyond their control. i.e. if some idiot to posts illegal content without their knowledge.
Of course once notified by the copyright owner that they are hosting infringing material, I suspect the forum owner would then become liable if they do not take the offending material down (in much the same way that ISPs are). It depends on the country of hosting of course, but I would also expect the ISP hosting the forum (once notified) to take the site down if the forum owner did not remove the illegal content, or if the forum had a reputation (or encourages) illegal content.