"ra" probably stands for "real audio". The real player was a popular format in the early years of digitizing music for online publication. No idea whether the player software is still around or can be decoded by other audio players.
The Hindemith is even more gorgeous fun when reading along with the illustrated version he made for his wife Gertrud:
http://www.mtosmt.org/issues/mto.10.16.3/mto.10.16.3.walden.html
The facsimile edition, btw, is still available from Hindemith's publisher Schott:
https://de.schott-music.com/shop/ludi-leonum-no38752.html
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That said - how about Shostakovich?
In any case it's an excellent training for any singer or instrumentalist to develop his improvisatory skills. I remember I once played along the Soul Station sax solos to try a different approach to conga drum soloing. Works fine.
The question is: how to present thematic material and then variate it. Most horn players or pianists are not that much better at this, just running the changes without much reference to the theme or its elements. You can play a solo by thematic variation or just run licks. The former is necessary for drummers or you will bore the listeners. Doug Hammond is great at this.
I have that Roach LP - really good, he clearly refers to Baby Dodds, Jo Jones, Big Sid Catlett, Coty Cole.
Doug Hammond referred to all these and Max, and did combine drums solos with recitations of his poetry.
He certainly doesn't sound like 81 on this recording. Soprano saxist Richard Hadlock has equal solo space. Bushell is an excellent player in the New Orlenas tradition with a personal style, not as individual as Pee Wee Russell or Edmond Hall, but nice to listen to. There are some bassoon solos as well. The recorded sound is excellent, a lovingly üroduced tribute like all the Delmark CDs with veteran players.
The Herreweghe recording is great, but listen to this one - stunning sound that places the listener right in fornt of the ensemble, and incredibly moving:
That's what I wanted to point out.
Even drummers have that "problem", that's why most add some melody instrument or vocals to outline the structure of a piece. Few drummers / percussionist have enough sense of form to transport that without melody isntruments and without slipping into displaying technical chops.
A great example for sense of form with drums only are Baby Dodds' 1940's drum solos. I will think of more examples.
It was an original recording for the small French Harmonic label. He also made a Debussy disc for them on the same piano. Ask the label if they still have copies: https://harmonicclassics.com - click on "inquiry" beneath the album cover.
One of my favourites is this Badura-Skoda recording on a one hundred year old Bösendorfer grand piano that sounds absolutely beautiful. He played that instrument as a boy and was able to acquire it from the owner's legacy. You can hear how much he loves that piano.