There are a number of ways to increase the volume in your Music settings. One of the easiest ways on an iPhone is by adjusting the EQ of Apple Music. Simply go to Settings > Music > EQ and select a different EQ. Most people suggest using the Late Night option to increase the volume of songs. You can also ensure that Lossless Audio is enabled by going to Settings > Music > Audio Quality and ensure that Lossless Audio is enabled. However, keep in mind you need a subscription to Apple's music streaming service to enable this feature.