libopus 1.3-26-ge85ed772 has a huge jump in quality on the 'noisy' sample at 9 kbps (CVBR or true VBR), because it moves from a 4 kHz lowpass to 8 kHz. In 2010 Nokia's listening test found [1] that SILK (at the time an independent speech codec, later incorporated into Opus) gained a quality benefit from reaching 8 kHz vs. the ITU-T and 3GPP codecs that would top out at 7 kHz for comparable modes and bitrates. So any gain in bandwidth in the 0-8 kHz range makes an appreciable difference, especially when there's distracting background noise in the lower bands and you can't filter it out so you have to encode along with your signal.

While Opus can go as low as 6 kbps, at that bitrate it very clearly sounds like narrowband audio we're used to from telephones. Frustrating, but not an unfamiliar kind of degradation.

Speex behaves like a classic CELP codec and will get robotic at its low end; 3 kbps is just a cruelly low bitrate for a codec whose advertised range is 2-44 kbps.

Lyra does sound richer and wider-band than both Opus and Speex, but there's also a peculiar style transfer going on that's most apparent to me in the chocolate bread sample. Opus clearly sounds like a low-quality encode of the original -- it would benefit from some background noise reduction prior to the encode.

But the Lyra version exaggerates the pronunciation of the phrase 'with chocolate' in a way that meaningfully differs from the speaker's original. It weakens the voiced 'th' to nothingness, and overshoots both the lead consonant and first vowel of 'choc', and then proceeds to wash the entire rest of the sentence with a peculiar brightened voice that's high, lacks consonant definition, and is close to ringing.

I'm guessing it's actually style transfer, because though the result sounds not much like the speaker's original, the result is reminiscent of the speech pattern and accent that people with East Asian and Southeast Asian ancestry adopt when speaking American English. It was surprising, given that the speaker doesn't sound like that in the original. Does anyone else hear this too?

[1] Rämö, Anssi & Toukomaa, Henri. (2010). Voice quality evaluation of recent open source codecs.