Singing songs should be an integral part of shakuhachi practice. I mean this literally,as in singing the names of the notes “Ro-Tsu-Re-Chi-Ri”. Of course, your voice should match the pitch of the note.
You don’t have to say “Ri-meri” when you come to Ri-meri, of course (it would be too hard to sing!), but you should still lower your voice half a notch lower than a normal Ri. You can even sing a whole song in one long “Ahh.....”, just as long as you modulate your voice properly.
Some people like to play along with CD’s. Instead, try singing along with CD versions of good players playing songs you like.
This is an especially good way to improve your honkyoku. After all, we are all more skilled at using our voice than a shakuhachi, so we can try out new variations of things like ma (timing), pitch, and volume more readily with our voice than with the flute.
Then we can incorporate the fruits of our experimentation, and the understanding of the song gained, into our playing. It doesn’t matter if you aren’t a very good singer at all - just start out humming.
If you reach the point where you can sing a shakuhachi piece the first time you see it - before you play - you will be very skilled indeed.