Comment 11 for bug 850662

If it helps,

the pulse client API crashes on

    pa_assert(channels > 0);

called from:

  pa_cvolume_set(&priv->volume, priv->channel_map.channels, new_volume_value);

I'm guessing priv->channel_map.channels is zero because it has not been initialized, i e slider_menu_item_update_volume is called before slider_menu_item_populate or slider_menu_item_update.