Personally I prefer NOT to use the XXX (e.g. 755) syntax, as you might overwrite permission flags that already were fine as they were... Besides calculation failures might happen..
The chmod command allows you to use a (from my point if view) better syntax which allows better reading and setting delta's (see the examples below for more clarity)
chmod [who][operator][permission] file/directory
(More syntaxes are possible but I wanted to keep it "simple" at first)
Who:
u User
g Group
o Other
a All
Operator:
- Take away permissions
+ Add permissions
= Assign permissions absolutely
Permission:
r read permission
w write permission
x execute permission
(And the special permissions that need a bit more knowledge:
l mandatory locking
s user or group set-ID
t sticky bit
X execute permission if the file is a directory or if there is execute
permission for one of the other user classes)
So if you want to give your mp3 files a read permission for the user (owner) without changing the other permissions you would do something like:
chmod u+r *.mp3
To remove write for Other:
chmod o-w *.mp3
To remote read/write/execute for other:
chmod o-rwx *.mp3
To add read/write for user/group and add read for other:
chmod ug+rw,o+r *.mp3
To SET (removes permissions when not in set) read/write for user/group and read for other:
chmod ug=rw,o=r *.mp3
For clarity, a chmod 770 would be:
chmod ug=rwx,o= *.mp3
and a cgmod 775 would be:
chmod ug=rwx,o=rx *.mp3
BETTER would be the following, as an execute bit on the mp3 doesn't make sense and it would only set the execute when it's a directory or already have it:
chmod ug=rwX,o=rX *.mp3
How this doesn't confuse some people, but calculating bits... I find it old fashioned!
Anyway, there is always the man page (man chmod) which will help too!!