I know I'm not the only base WRX owner to experience this simply from forum discussions on NASIOC and elsewhere. The stock volume on the base head unit goes up to 38 and no matter what the source is (radio, aux in, bluetooth, in-dash apps), whenever you get anywhere 34-38 on the volume dial then you start to experience random speaker brownouts where they go quiet. It's very random and jumps from speaker to speaker. I've had it end up where only one speaker is playing and it's (for instance) the passenger front one alone.

Simply bringing the volume down slightly almost immediately allows most/all speakers to return to working normally. This suggests to me moreso than overheating that most likely either the wiring to carry the current is insufficient or more probably the amps running those speakers are underpowered to not even be able to power them to the full max volume listed on the dial.

It's incredibly annoying and needs to be examined and TSB'd by Subaru.