Legally you don't have to solve the problem and you can continue to transmit. But in the interests of neighborly relations it is nice to help solve such a problem. It could help to let the neighbor know this.
The simple (partial) solution is for the neighbor to shut his powered speakers OFF when not in use.
These links might help the neighbor understand. http://www.fcc.gov/guides/interference-defining-source http://www.arrl.org/regulatory-rfi-information
From the ARRL website link.
"Interference caused by insufficiencies in consumer equipment
In some cases, however, RFI is caused by the fact that most consumer equipment
lacks the necessary filtering and sheildng to allow it to work well near a radio transmitter.
The FCC does not regulate the immunity of equipment, however, so when interference is
caused by consumer-equipment fundamental overload, there is no FCC rules violation,
and licensed stations have no regulatory responsibility to correct interference that may
result. (Hams may want to help find a solution to be neighborly, but this is not a requirement in the rules.)"