For clarification, the following is a guide on getting Music Titles to appear for music added with Dantarion's Custom Sound Engine code. By "Music Title" I mean the name of the song that appears on the top-left of the screen when a battle starts. This guide will not modify anything on My Music, to avoid corrupting Save Files. As far as I have tested, this guide does not corrupt save files. Occasionally, I run Vanilla Brawl and mess with My Music to make sure nothing's messing up. Nothing has yet, but if you are worried back up your save beforehand. This method has only been tested on the US (RSBE) version of the game, and is not guaranteed to work in other regions.
This is the pre-made method of inserting Song Title slots. Since this is most likely only useful for those with both the Custom Sound Engine and Stage Expansion codes active, I am going to assume you are comfortable with basic file placement and naming.
First, download this.
In the distributed .rar file, there should be two files: info.pac and common2.pac. If you are using Riivolution, rename the files to info_en.pac and common2_en.pac respectively. Place info(_en).pac in /pf/info2/, and common2(_en).pac in /pf/system/. These two are pre-modified to give Title Slots to files named 0000C000.brstm to 0000C25F.brstm. That is enough to give 16 songs on all 37 expanded stages a custom Title, so it should satisfy most users.
BRAWLEX USERS: Though I have not tested it personally, you should be able to export the MiscData from my Common2 and import it into Phantom Wings' modified Common2 using Brawlbox. I highly doubt that the MiscData in his Common2 contains any necessary modifications.
Once placed properly, you can edit info.pac just as you would replace any existing song title. The Song Titles that I have added are appropriately named for easy searching. (0000C14F.brstm's Song Title is 'C14F' for example.)