How can one use music lumps from an IWAD for a custom mapinfo lump?

[sacarias]

Basing on this guy's initiative:
viewtopic.php?p=1106966#p1106966
I started playing with his suggested pk3 for learning purposes.

Loading it as just any PWAD as suggested there, it "works", but not with the correct music for the level -among other things-. I'm loading it as PWAD, but always with "-iwad DOOM2.WAD"
I learned that the "$MUSIC_XXX" variables are referencing the language.def core lump, so in the custom mapinfo.txt lump I changed, for example,
music = "$MUSIC_DDTBLU"
for
music = "ddtblu"
Still no luck, console says "Music ddtblu not found".

What am I missing? Could someone help?
Thanks.

[sacarias]

Nevermind, I think the actual problem was how I was packing everything into PK3...

If packing like this: mapinfo.txt graphics/titlepic.png NERVE.WAD, everything works (even all GZDoom core lumps seem to be inherited or the like).
But if packing like: mapinfo/mapinfoNerve.txt graphics/titlepic.png maps/NERVE.WAD (in order to prevent RAM leaking according to the docs), this is totally broken.

Why does second way break things?

[Graf Zahl]

You have to remember one thing about Doom.exe:

Doom.exe adds a 'd_' prefix to all music it plays, the strings do not have the full file name. As this is what Dehacked modifies, the names in the externalized string table also come without the 'd_' prefix which gets prepended when the string is substituted. This is necessary for Dehacked string substitution logic to work as intended.
GZDoom does not do any special treatment for the 'd_' prefix outside of Dehacked, so when directly using that music, it's called 'd_ddtblu', not just 'ddtblu'.

[sacarias]

Thanks for explanation.

Now, for last, could you help with the PK3 packing other issue?