游戏音效渐变

车游戏，需要在开始的时候音效小，后面的时候音效大。比如跟车速有关，开始小车speed为0 ，音效为0，随
着speed增大，音效的声音也随着增大。

soundpool的pitch方法
Changes the pitch multiplier of the sound instance with the given id as returned by {@link #play()} or {@link #play(float)}.If the sound is no longer playing, this has no effect. @param soundId the sound id @param pitch the pitch multiplier, 1 == default, >1 == faster, <1 == slower, the value has to be between 0.5 and 2.0

可以使用下面的方式来控制。

sound.setPitch(carBody.getLinearVelocity().len() / 19.0F);