as far as I know there are specific plugs for this use, that have a certain curve in the blocked frequency range. regular plugs will do though, to the best of my knowledge, except you might miss out unnecessarily on some of the music (the lower end I think) since they’ll block more evenly. something like that. put another way, there are plugs that avoid muffling

Bluetooth audio is subject to a bunch of details. Might be worth checking the configuration. Also the XM’s are likely a lot more amenable to equalization, so it’s worth at least experimenting. Even just the default audio settings options on my phone produce very different results depending on what is selected. I don’t have XM’s though.