That's a known issue with the current version of WadSmoosh, I need to figure out how to properly support the widescreen graphics that are now included with GZDoom. In the meantime, I'd suggest just replacing the graphic in the pk3, and editing the GameInfo section of the pk3's mapinfo.txt to use a different TitleMusic.
In the top level of the PK3's hierarchy, look for the file called "mapinfo.txt". You should be able to edit it right in SLADE. Look for the line in the GameInfo block called TitleMusic. Replace "$MUSIC_DM2TTL" there with whatever music track you want. ($MUSIC_DM2TTL is a localized string but everything after the underscore should correspond with the name of a music lump you can see in the vanilla IWADs.