For anyone facing similar problem I found a solution. The background track has an AVAudioPlayer which has a rate property. The default value for the rate float is 1 (normal speed). You can change it to slow down the playback (value lower than 1) or speed it with values higher than 1.
There is one important thing, you have to set the BOOL property enableRate to YES before the player starts playing (otherwise rate won't work). You can make it in the play method of OALAudioTrack.m file between [player stop] and [player play] commands.
Here is the example of changing the rate property:
OALSimpleAudio* audio = [OALSimpleAudio sharedInstance];
[audio playBg:@"your_file.wav" loop:YES];
audio.backgroundTrack.player.rate = 0.5; //slow down the playback