Steps on how to reproduce:
1. Have two players join lan world.
2. Insert this command into a command block:
execute @a ~ ~ ~ /playsound entity.generic.explode blocks @p
3. Execute the command, and observe guest player in the LAN world get kicked from the server (not the LAN host) with the error message:
"Internal Exception: io.netty.handler.codec.DecoderException: java.lang.IndexOutOfBoundsException: readerIndex(X) + length(1) exceeds writerIndex(X): UnpooledHeapByteBuf(ridx: X, widx: X, cap: X" where capital X is the length of the sound played + 2.

Note that whilst I managed to produce the error with two accounts on the same computer (on both clients), it also happened to my friend on my LAN server, causing him to crash.

Attached crash log.