This is Gordon R Dickson's In The Bone, first published in If: Worlds of Science Fiction, October 1966.
Here's the alien pyramid-like ship:
The world was one which, from orbit, seemed to be the best of all the
planets which he had discovered were suitable for human settlement;
and he was about to go down to its surface personally in the
control-suit, when his instruments picked out something already down
It was a squat, metallic pyramid about the size of a
four-plex apartment building; and it was radiating on a number of
interesting frequencies. Around its base there was mechanical movement
and an area of cleared ground. Further out, in the native forest, were
treaded vehicles taking samples of the soil, rock, and vegetation.
And here's the message:
The lightning vanished. A yellow lightness filled the air about Harry
and the dismembered suit. There was a strange quivering to the
yellowness; and Harry half-smelled, half-tasted the sudden, flat bite
of ozone. In the headpiece a button clicked without being touched; and
the suit speaker, still radio-connected with the recording tank in
orbit, spoke aloud in Harry’s voice.
“Orbit …” it said. “… into … going…”
These were, in reverse order, the last three words Harry had recorded
before sighting the pyramid. Now, swiftly gaining speed, the speaker
began to recite backward, word for word, everything Harry had said
into it in nine weeks. Faster it went, and faster until it mounted to
a chatter, a gabble, and finally a whine pushing against the upper
limits of Harry’s auditory register.
Suddenly, it stopped.
The little clearing about Harry was full of silence. Only the odd and
distant creaking of something that might have been a rubbing branch or
an alien insect came to Harry’s ears. Then the speaker spoke once
“Animal …” it said flatly in Harry’s calm, recorded voice and went on
to pick further words from the recordings. “… best. You … were an
animal … wrapped in … made clothing. I have stripped you back to …
animal again. Live, beast …”
Almost everything else in your description matches up as well. After being stripped of his technology, the main character suffers what today would be called a psychotic break. Eventually he's able to start getting himself together enough to defeat the alien.