Where do you get Minecraft sound effects packs?

They're on Minecraft Marketplace, downloadable since November 10. Marketplace is a Bedrock feature, so sound packs are Bedrock only: Windows, console and mobile. Java doesn't have them.

Minecraft Marketplace sound effects packs key art
Image: Mojang Studios / Microsoft
Source / rights
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Short answer

Minecraft now has packs that reskin the game's audio: zombies, creepers, blocks and music all get new sounds. Seven packs were spotlighted on day one, and the Wilderness Bound: Haunts & Harvest pack is free.

What do sound effects packs change?

Per Mojang, packs swap mob, block, combat and ambient sounds plus music. One pack turns the zombie groan into a strangled roar; another makes creepers go off like a cymbal crash. You can share your remixed world on Realms with friends (Realms subscription required).

Which sound packs are out?

Minecraft.net spotlighted 7 packs on day one:

The first 7 sound effects packs
PackCreatorStyle
ToonCube: Cartoon Sound Effect Pack Cube Games Cartoon sounds; creepers go off like a cymbal crash
Liminal Sound Pack Panascais Uncanny horror mood with sparse piano
Retro Arcade Sound Pack King Cube 80s arcade bleeps and synth music
Midnight Carnival — Circus and carnival tunes
Super Satisfying Sounds — Crunchy, satisfying sounds
Jigarbov Sound Pack — Jigarbov's own sound set
Zephyr - Relaxing Sounds Pixelbiester Softer, mellower vanilla sounds

Each pack shows its price on Marketplace; Mojang didn't list prices in its post.

Is there a free sound pack?

Yes: Wilderness Bound: Haunts & Harvest is free. Made by Mush Co, it adds 100+ fall sounds, and Mojang announced it on October 28. For the free add-on you can earn during TwitchCon: Minecraft Toying Around free add-on.

Mojang's official Sound Effects Packs video.