Check:
Settings → Music → Volume Limit = Off
You can also try:
Settings → General → Accessibility → Hearing Devices → Hearing Aid Mode = On (this sometimes boosts volume)
You can also try:
Settings → Music → Sound Check = ON (this can compensate for tracks in low volume).
Note: Be careful with high volume, it can damage your hearing. Listen responsibly: https://www.apple.com/sound/