When your roommate does not consider your external costs in the absence of external costs from playing music, the number of hours played is

the number of hours music played is too high

Explanation:

When the roommate does not consider the external cost in absence of external benefit then the hours of music played is too high. The roommate has bought the expensive stereo system so his marginal benefit is lower than the marginal costs. The marginal benefit will equal to the marginal cost when the stereo system is played for many hours.

answer; break-even analysis;