Audacious

Do you have the C64 ROMs installed? There should be three files ("basic", "chargen", and "kernal") in /usr/share/sidplayfp.

No, I don't have any of those files installed.

And I can't find which package would supply them.

~$ apt-cache search --names-only sidplay
libsidplay1 - SID (MOS 6581) emulation library
libsidplay1-dev - SID (MOS 6581) emulation library (development files)
libsidplayfp-dbg - library to play Commodore 64 music based on libsidplay2 (detached symbols)
libsidplayfp-dev - library to play Commodore 64 music based on libsidplay2 (development files)
libsidplayfp-doc - library to play Commodore 64 music based on libsidplay2  (documentation)
libsidplayfp3 - Library to play Commodore 64 music based on libsidplay2
sidplay - Music player for tunes from C64 and C128 (console)
sidplay-base - Music player for tunes from C64 and C128 (console)
libsidplay2 - SID (MOS 6581) emulation library
libsidplay2-dev - SID (MOS 6581) emulation library
sidplayfp - Fork of sidplay2, a C64 and C128 music player
~$ dpkg -l *sidplay*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                                  Version                         Architecture                    Description
+++-=====================================================-===============================-===============================-================================================================================================================
ii  libsidplay1                                           1.36.59-7                       amd64                           SID (MOS 6581) emulation library
un  libsidplay1-c102                                      <none>                          <none>                          (no description available)
un  libsidplay1.36                                        <none>                          <none>                          (no description available)
ii  libsidplay2                                           2.1.1-14                        amd64                           SID (MOS 6581) emulation library
un  libsidplay2-1                                         <none>                          <none>                          (no description available)
un  libsidplay2-1c102                                     <none>                          <none>                          (no description available)
un  libsidplayfp                                          <none>                          <none>                          (no description available)
ii  libsidplayfp-dbg                                      1.7.0-1                         amd64                           library to play Commodore 64 music based on libsidplay2 (detached symbols)
ii  libsidplayfp-dev:amd64                                1.7.0-1                         amd64                           library to play Commodore 64 music based on libsidplay2 (development files)
ii  libsidplayfp-doc                                      1.7.0-1                         all                             library to play Commodore 64 music based on libsidplay2  (documentation)
ii  libsidplayfp3:amd64                                   1.7.0-1                         amd64                           Library to play Commodore 64 music based on libsidplay2
ii  sidplay                                               2.0.9-6                         amd64                           Music player for tunes from C64 and C128 (console)
un  sidplay-base                                          <none>                          <none>                          (no description available)
ii  sidplayfp                                             1.3.0-2                         amd64                           Fork of sidplay2, a C64 and C128 music player
~$ ls -l /usr/share/sidplayfp
ls: cannot access /usr/share/sidplayfp: No such file or directory
~$ apt-file search /usr/share/sidplayfp
~$ 

They are proprietary firmware, so it may be illegal for you to download them, depending on where you live. However, you can find them at http://sourceforge.net/p/vice-emu/code/HEAD/tree/trunk/vice/data/C64/.

I currently live at Greece.
How to tell whether it's legal or not for me to download them?
And what do, if I can download them?
Place them in the path you mentioned?
How would these files help me debug audacious' behaviour?

I know nothing of Greek law, and I wouldn't give you legal advice on this forum even if I did. But from a technical standpoint: there is nothing to debug. Copy those three files into /usr/share/sidplayfp and restart Audacious. Then the song will play just fine.

Indeed it did.

Only drawback I don't know why.

Last and not least, thanks! :D

Unlike old libsidplay, its successor libsidplay2 and the even newer libsidplayfp contain an emulator engine that is more powerful with regard to executing Basic and Kernal ROM machine code in order to run programs written in Basic, for example. It's a good thing that libsidplayfp does not include the ROM image files anymore, or else various distributors of prebuilt packages could not offer the software due to legal concerns.

The default location where to store the image files is explained in "man 5 sidplayfp.ini", since it's the player frontends (such as "sidplayfp") that need to load the optional ROM images into libsidplayfp.

Why libsidplayfp doesn't attempt at loading any ROM image it can find in a list of default places, I don't know. That would be the more convenient solution to the problem, because it could learn about where VICE stores those files. Perhaps open a ticket at the libsidplayfp site and request that feature?