Yeah, a lot of the "details" are 8khz and up. A pure 15khz tone isn't fun to listen to (if you can hear that high) but if you cut off everything above 8khz (or 7khz, or etc) things will start to sound increasingly more muffled. Experiment in Audacity or such to see for yourself.

No one wants to listen to pure tones over 8kHz, but things like cymbal crashes would sound rather different if you cut off those higher frequencies. The primary frequencies are of course under that cut-off, but there's lots of other frequencies above it.

The harmonics of a guitar or piano can extend well beyond 8kHz, even though they contain lower frequency components that give them their characteristic sound. In fact, the higher harmonics can contribute to the richness and complexity of the overall sound, even if they're not heard as distinct pitches. Similar effect occurs with cymbals, the high-frequency shimmer of cymbals can extend well above 8kHz. Also violins can include overtones that extend beyond 8kHz and the sound of a trumpet can include overtones that extend up to 10kHz or higher.