To arrange music you must seek "Permission to Arrange" from the company or individual that holds the "print rights" to the music. The "print rights" holder may or may not be the same as the "copyright owner." The copyright owner sometimes publishes its own music or it may grant "print rights" to a music publisher to arrange their music for band. Copyright owners often empower the music publisher with the right to grant or deny requests for permission to arrange. You will seek permission from either the copyright owner or the music publisher, depending on their contract. The Composition and Publisher Databases are designed to assist in finding the "print rights" holder.
Permission to Arrange is usually granted to specific arrangers, for specific performances, by specific bands, and usually for a given year. Furthermore, contracts between copyright holders and music publishers can change from year to year. A license granted in the past does not guarantee that it will automatically be granted this year. Your approved arrangement is generally the property of the original copyright owner not the property of the band or the arranger. You may not loan or sell your arrangement to another band without first obtaining permission from the "print rights" holder. Additionally, you may not purchase an arrangement from another band or arranger without first acquiring permission from the copyright owner or music publisher. In addition, some companies may require that you forward scores and/or parts as a part of your contract.
The following is a good explanation of mechanical licenses: http://www.vochor.com/mechanical_licenses.html
Here is some additional info for the music industry & copyrights:
ASCAP (The American Society of Composers, Authors, and Publishers) is the primary source for licensing of music. Go to the section on licensing for information on licensing radio, television, and Internet materials as well as general licensing.
BMG owns Arista Records, RCA, and BMG Music Publishing..
DiMA (Digital Media Association) represents webcasters, technology companies, and online music and video retailers.
EMG Music Clearance represents numerous songwriters and will negotiate licensing.
EMI holds the rights for EMI Music Publishing and EMI Recorded Music. Searchable database of their music labels.
Harry Fox Agency is a clearinghouse to license musical works. You can search the songfile of the National Music Publishers' Assocation.
Parker Music Group: Music Clearance will obtain clearances for songs or master recordings.
Recording Industry Association of America has a good discussion of Web Licensing.
Sesac represents songwriters and publishers.
Signature Sound is another clearinghouse that will obtain music licenses.
SONY owns Columbia Records.
Time Warner owns HBO, CNN, Warner Brothers, Warner Music, Turner Broadcasting, and Time.