The basic MP3 decoding and encoding technology is patent-free in the European Union, all patents having expired there by 2012 at the latest. In the United States, the technology became substantially patent-free on 16 April 2017 (see below).
If the longest-running patent mentioned in the aforementioned references is taken as a measure, then the MP3 technology became patent-free in the United States on 16 April 2017 when U.S. Patent 6,009,399, held by and administered by Technicolor, expired.
The three exceptions are: U.S. Patent 5,878,080, expired February 2017; U.S. Patent 5,850,456, expired February 2017; and U.S. Patent 5,960,037, expired 9 April 2017.
DAC2's USB 1.0 mode doesn't work with Linux kernel 3.8.x, but USB 2.0 does. So switch it to USB 2.0 and forget about it. Next, I figured that the audio was simply being muted in some instances. The solution is to open a terminal window, run AlsaMixer, hit F6 to configure the DAC 2. Even though their levels cannot be altered (they're fixed at 00), the first two items must be unmuted in order to get any sound:
如同文章裡說的，USB 1.0 模式 (會抓到「Benchmark DAC2 USB Audio 1.0」) 是不會動的，而 USB 2.0 可以 (會抓到「Benchmark DAC2 USB Audio 2.0」)，但播放時沒有聲音。
解法則是使用 alsamixer，進入後按 F6 選 Benchmark DAC2，將前面兩個用 m 給 unmute 掉就好了：