If the volume of the speaker of your mobile phone is too low, then it can be increased.
All you need to do is find a broad neck vessel. Place the phone with music on in such a vessel, with the speaker facing the ground.
This will make the sound echo in the vessel and with a broad neck of the vessel, the sound which will reach to us will be multiplied and hence the volume of the music played will be increased.
A paper cup, jug, coffee mug etc. can be used. Make sure they have broad necks.