Teensyduino 1.52 has been released.

https://www.pjrc.com/teensy/td_download.html

1.52 brings support for Teensy 4.1, USB audio on Teensy 4.x, OctoWS2811 support for Teensy 4.x with LEDs on any number of pins, USB dual & triple serial, 4/6/8 channel I2S on Teensy 4.x, RGBW support in WS2812Serial, and many small improvements and bug fixes.

Changes since Teensyduino 1.52-beta6:

Workaround for USB memory bandwidth at very slow CPU speeds
Fix import bug in Audio Design Tool (Frank B)
OctoWS2811 fix memory use with fewer than 8 pins
Clean up compiler warnings in USBHost_t36 ADK driver

The full list of changes since Teensyduino 1.51:

Support for Teensy 4.1 USB Audio support for Teensy 4.0
OctoWS2811 ported to Teensy 4.0
WS2812Serial support for RGBW SK6812 LEDs
FreqMeasureMulti ported to Teensy 4.0 (KurtE)
Dual & Triple USB Serial (Geert Uytterhoeven)
Audio support for 4,6,8 channel I2S on Teensy 4.0
Audio support for SPDIF input on Teensy 4.0 (FrankB)
Audio DAC support for Teensy LC (Ben Kurtz)
Fix Serial.print() conflict with USB Touchscreen
Fix contact count on USB Touchscreen (purple-ben)
Defined FLASHMEM on Teensy 3.x, for compatibility with Teensy 4 code (FrankB)
Fix cache handling in I2S audio input on Teensy 4.0 (Frank B)
Fix numerous iMXRT register field defines
MTP USB descriptors on Teensy 4.0 - actual MTP support still experimental
Libraries updated: FlexCAN_t4, ST7735_t3, RA8875
Fix USB RawHID on Teensy 4.0 (KurtE)
Fix USB audio+midi+serial descriptor on Teensy 4.0 (widdly)
Add ADK to USBHost_t36 (Matt Bergman)
Fix USB Host MIDI transmit race condition (essej)
Optimize USB interrupt on Teensy 4.0 (Frank B)
Arduino compatibility of digitalWrite control pullups (FrankB)
Add missing MPU traps on Teensy 4.0 (FrankB)
Fix SGTL5000 clock ratio setting (FrankB)
Fix I2S slave on Teensy 4.0 (widdly)
Add DMAMEM & fix warnings in I2S on Teensy 4.0 (FrankB)
Improve SPI use by SD on Teensy LC (rutmarti)
Support alternate SPI pins on Teensy 4.1 (KurtE)
Support alternate Teensy 4.1 CS pins on ILI9341_t3 (KurtE)
Fix Teensy Loader on MacOS 10.13 with Catalina version
Fix teensy_serialmon 100% CPU on Linux if Arduino process killed
Workaround for rare Windows Defender flase positive when installing Java classes
Allow SD.begin() to be called more than once
Support hardware serial CTS pins on Teensy 4.1 (KurtE)
Fix double float promotion (Frank B)
Fix I2S input use of cache (Frank B)
Fix installer crash on Jetson Nano
Update libs: RA8875, ILI9844_t3, QuadEncoder, ST7735_t3
Fix RTC auto-set on Teensy 4.x with daylight savings time
Fix RTC set on Teensy 4.0 (Silverlock)
Audio ADC input on Teensy 4.0 - experimental
Hardware serial pin fixes for Teensy 4.1 (KurtE)
Add dummy Keyboard.h on Teensy 4, for Arduino compatibility
Use standard (not low power) ADC mode
Align DMA buffers to 32 bytes (Frank B)
Fix ADC_ETC defines
Use recommended analog bias current on Teensy 4.x (Frank B)
Update Snooze library
Fix Audio ADC input on Teensy 3.x
Initialize PSRAM chips at startup on Teensy 4.1
Increase PSRAM clock to 88 MHz & fix error in writing clock setting
Use strong drive & hysteresis for FlexSPI2 pins
Minor improvement in OctoWS2811 for Teensy 4.x
Workaround for USB memory bandwidth at very slow CPU speeds
Clean up compiler warnings in USBHost_t36 ADK driver
Fix import bug in Audio Design Tool (Frank B)
OctoWS2811 support for any group of pins on Teensy 4.x
OctoWS2811 fix memory use with fewer than 8 pins