The Taiwan Wren Babbler (Taiwan Cupwing) (Pnoepyga formosana), endemic of high montane forest, is a tiny, fast moving bird, usually in the dark undergrowth; making for challenging observations. Very much like the Malaysian Pygmy Wren-babbler (Pnoepyga pusilla). Handbook of the Birds of the World (2019) describes both a dark and a pale morph (Post 1-3 would be a pale morph) but I am not able to find similar descriptions in other references.
I managed to record the song and an edited recording is here: https://soundcloud.com/amar-singh-hss/taiwan-wren-babbler-calls-3-1-daxueshan-national-forest-recreation-area-taichung-city-county-taiwan-17th-january-2018
Brazil (2009) rightly describes the song has having a first note separated from the rest – this is heard well when you record and play back the song but can be missed in the field. The song comprises 6 notes. The first note that starts off the song is not well seen on the sonogram but clear on the waveform (Post 4). It is followed by a rapid sequence of 5 notes that have both a high frequency and low frequency sound (quite complex). The song last approximately 1 to 1.2 second (but I measured a number of them; Handbook of the Birds of the World says 1.5 seconds). The song is repeated 6-26 seconds apart (spacing 13, 18, 26, 13, 10, 9, 6, 9 seconds on record).
Amar-Singh HSS (Dato’ Dr) – Ipoh, Perak, Malaysia
Location: Daxueshan National Forest Recreation Area, Taichung City County, Taiwan
Habitat: 1,750-2,500 meter ASL, forested region
Date: 15 & 17th January 2019
Equipment: Nikon D500 SLR with Tamron SP 150-600mm f/5-6.3 Di VC USD, handheld with Rode VideoMic Pro Plus Shotgun Microphone