Maybe somwthing in BSplayer folders/settings got broken.
Uninstall BSplayer and delete its folders, by default:
C:\Program Files\Webteh\BSplayer\
%AppData%\BSplayer\

Then reinstall BSplayer and try to play those files again. At first, don't download suggested codecs, you can download them later by starting the executable located in C:\Program Files\Webtwh\BSplayer\codecmanager.exe

Please, post here the results

Unistalled BS player
Manually deleted folders on "documents and settings" and in "program files"

Installed BS Player v2.50.1015 Beta with No codecs during installation
-> mp3 still not working

Trying to install codecs using "codecmanager.exe"
(it says not installed at all codecs)
selecting all codecs and hitting "Install selected"
Downloading -> Operation Complete
Green "Installed" at all codecs ->hitting Go to BSplayer
Trying MP3 file -> Could not find .... (same thing as i posted)

Not solved :(

EDIT: WinAMP and WMP11 still plays same MP3 file with no problem

Try to update to latest release (v2.50.1017), then try to re-register quartz.dll

START -> Run... -> regsvr32 quartz.dll

Updated

"Provedení DllRegistrerServer v quartz.dll se zdařilo"

(Czech language) -> re-registered succesfully

now it playes MP3 files finally ... but other problem has come around, seems like the:
libavcodec.dll is broken somehow

when i play now .avi file it immediately gives me Error to Close BSplayer "acces violetion at adress ...." in libavcodec.dll

so i redirected in FFDshow to Xvid codec and others and it seems working but in some files there is only choice of lbavcodec.dll.
Can you pls post some download links directly to download it or is it problem in entering data from BSplayer ?

ok, downloaded:
Free Download DLL and Support Forum

the old one had
4 835 652 b
the new one
611 441 b

looks like solved
Thx for help

Happy long-term user of BSplayer :)