Are there really any difference to opus audio files and for example m4a files detectable to human ears?

The standard way to store audio information digitally is pulse-code modulation (PCM)[1]. This consists of a sequence of samples representing the amplitude of the signal at different moments. For example, say you record yourself speaking into your computer's microphone. Your voice causes a fluctuation in air pressure, which the microphone converts into a fluctuation in voltage. Your computer's sound card[2] then checks the voltage 44100 times per second (assuming CD quality audio) and stores all those values. Playback does the same thing in reverse. Your sound card converts the samples back to voltage levels, which your speakers convert back to air pressure, which your ears detect and perceive as sound.

The quality of the audio depends on the quality of your sound card, the sample rate (44100 Hz is standard for CDs, but you can go higher), and the bit depth[3] (basically how much we round the sample values). However, once the signal is converted to samples, the audio quality will stay the same as long as those samples are not modified. wav and aiff files are PCM formats. They sound identical, because they store the exact same information. Nothing is going to sound better than a PCM format.

However, wav and aiff files are huge. It is often desirable to compress them. flac[4] is an an example of a "lossless" compression format, which means that although it compresses the data, it does so in a way that the original data can be exactly reconstructed, so the quality of a flac file is just as good as wav or aiff, but it is a smaller file.

A flac is still pretty big, though. We can get even smaller if we compromise a bit on quality. "Lossy" compression, as opposed to "lossless" compression, compresses the data in a way that not all of it can be recovered. Lossy compression algorithms try to only throw out details that humans are unlikely to miss, but there is a tradeoff between file size and quality. The better a compression algorithm is, the better quality it can retain with the same file size, or the smaller a file size it can achieve with the same quality.

Here is a comparison of a recording as a wav file, and then compressed to the same size with both mp3 and ogg vorbis.

https://en.wikipedia.org/wiki/File:Test_ogg_mp3_48kbps.wav

To my ear, the wav sounds the best, as expected. The ogg vorbis loses some details, but still sounds decent. The mp3 sounds like crap to me. It sounds muted, because it is missing some of the higher frequencies. If you can't hear this, you can see it in a spectrogram. I downloaded the file and used Audacity to generate the attached image. It is a plot of frequency over time. Notice how the third segment (mp3) has all those empty spots at the top representing the higher end of the spectrum, and how the second segment (ogg vorbis) has those mostly filled in, but still looks a little different from the first segment (wav).

So to finally answer your initial question, lossless formats all sound the same, but lossy formats sound different from each other and not as good as a lossless format, and yes, humans can tell the difference. I haven't specifically compared opus and m4a, though.

[1] https://en.wikipedia.org/wiki/PCM
[2] https://en.wikipedia.org/wiki/Digital-to-analog_converter
[3] https://en.wikipedia.org/wiki/Audio_bit_depth
[4] https://en.wikipedia.org/wiki/FLAC

"Are there really any difference to opus audio files and for example m4a files detectable to human ears?"

The primary difference isn't quality-related: Opus is a royalty-free codec, meaning that it's free of patent problems. So if you can't hear a difference, use the patent-free one. :)

I can't tell the difference between opus and AAC using my Rockbox player (11 years old Creative Zen).

But opus is still not commonly supported by the official (often proprietary) firmware of latest Hi-Fi players. I can't test it on those players. I also won't buy any new player, because "Secure Boot" is already well developed (by Tivo, Microsoft, etc.).

As I understand it, Opus is a replacement for both Vorbis and Speex (lossy codecs for music and speech respectively). Opus allows for a variable bitrate, so if you have song where the music stops in the middle, and there is only someone singing a chorus (or whatever), the bitrate is dropped for this bit. The benefits of Opus are a) you get smaller files for the same or better sound quality, and b) you don't have to think about which bitrate to choose, the Opus encoder will do that for you.

As for the lossy vs. lossless thing, I think it depends very much on a) what kind of music are you encoding, b) what kind of speakers are you going to play the resulting files on, and c) how much do you care about how much storage space the files take up?

* If you are encoding the Sex Pistols, the Dead Kennedies, or the Tall Dwarfs, audio quality isn't the point, and the music already sounds like it was recorded with the microphones dipped in a bowl of soup. No point using a lossless codec for stuff like that. If you are encoding classical or dub, or other music where subtle differences in the sound make a big difference to the emotional impact of the music, it might be worth using a lossless codec like FLAC.

* If you are encoding any kind of music to listen to on headphones or computer speakers, even the best quality ones, you won't hear the difference between files encoded with lossy and lossless codecs, just use a lossy codec like Opus. If you plan to start DJing, and you hope to play the files on a huge sound system at an outdoor festival, then it's worth having your collection in FLAC.

* If you have near-infinite amounts of storage space, you might as well use a lossless codec anyway, just in case you get a chance to play the files on a bigger rig, or you're sharing them with someone who might. You can use the FLAC files to encode lossy files for use on portable players, and in other situations where you need smaller file sizes.