Texas has a noise ordinance that limits the volume to 60 decibels (roughly the sound you would hear in a busy restaurant). This ordinance only applies between 10 p.m. and 7 a.m., however.
In reality, it is not illegal to play loud music while driving in Texas. The problem is that many drivers get distracted and commit more driving errors and violations when listening to loud music. If a driver is caught speeding or blowing through traffic signals, he or she can expect to be cited. Additionally, if that driver causes a crash, he or she would likely be liable for the damages.
If a driver is cited for a traffic violation and is caught playing loud music in violation of the Texas noise ordinance, he or she could be fined an additional $500.