To clarify, I feel that music has a limited power in directly causing political change, but I think that political change over the past century or so, and perhaps even longer, has gone hand-in-hand with music.
It is always evident, at protests and rallies, the presence of music and songs. Music and lyric have long been interconnected with the notion of free speech. Songs are use as a means of conveying the message of the protests, becoming perhaps a part of a protests identity.
An example of popular music arguably causing political change, is the Live 8 concerts. These global events, indeed highlighted important issues, but caused no real noticable change.
Politics and music have a way of combining. Music is perhaps a route through which political issues and topics are brought to the fore, highlighting them, but never really effecting change. The question of whether highlighting the issues, is contributing towards political change is very relevant. Music indirectly causes some political change, making it a genuine force.