One of my favorite record stores in the 70s was this little place in San Jose's Valley Fair, back when it was a sleepy open air mall. It only sold singles, and had the best selection in the valley. Even Tower Records couldn't match it.
What made it especially popular was that it still had the old fashioned listening booths from the 50s. You could take a singles into one, put it on a record player and listen before buying. You couldn't do that with vinyl LPs, of course, but 45s were different. Singles were tougher, made to be stacked higher than shoes in a closet, and could be abused with sandpaper and still play...which was about what the average kid did to a 45.
The digital age has brought that back, more or less. An mp3 never will wear out, so it can played endlessly in sample form before purchase. Record companies have never objected to this, otherwise there wouldn't be radio...but for a while, it wasn't possible to preview a song over the internet (at least easily). The concept of a compressed file helped, of course, but early 30 second samples often bogged down the older computers (if the software could even play a sound file). Nowadays, a 60 or 90 second segment is standard, and on sites like Myspace Music, you can hear the whole track.
The barrier to making sure you hear a good enough sound to fully appreciate the music is now moot. Most computers have speaker systems that make my old stereo sound like a phone speaker, and the average iPod type device puts out better sound than many old time headphones.
When you're dealing with a sample, though, there is the question: does the segment being played really show you what the song is really about? Many of the services now use excerpts instead of just starting the sample from the beginning of the song. Which can defeat the whole purpose of a song intro, which was to draw the listener in.
iTunes, for example, plays 60 second samples, which seem to be geared towards showcasing the most "interesting" part. One of my songs, for example, "Beach Dog," starts right when my mandolin part starts to climb in the progression with a minor passage. We added that part to put a wistful feel into the number, and it's exactly the part I would want someone to hear (as long as you're not going to hear the intro that was meticulously planned to make the listener involuntarily click the "buy" button).
On the other hand, Amazon also uses a excerpt, but the selection process seems haphazard...sort of like putting the phonograph needle down on a random part of the record. One of the Handa-McGraw & The International songs, "Trouble In Tucumcari," opens with a drum part that's designed to draw you in and make you dance your butt off baby, but the sample comes in later, and even I felt a little confused about what that piece was about.
This can be a problem with jazz...most jazz songs state a theme and then improvise from there, and it's common to go into a dissonant section to add tension before moving back to the main melody. Get a sample that has that part, and it can make the listener think it's a free jazz piece.
Blues...even more so...particularly with old Delta Blues. Many were essentially improvs over a rhythmic vamp, much like a modern rap song, and you really need to hear the story in the lyrics in many cases. Luckily, most were also technically mood songs also, so a sample can do a good job of communicating the feel.
Unless you can hear the whole song, it's never certain if it'll please your tastes. But there is important information that can be gleaned from even a 60 second listening.
Even on a computer speaker, mainly used for beeps and other notification sounds, it can be obvious if the song is well produced (for some, that's important) and a good match for what it'll actually be played on. If it sounds good on computer, it'll probably sound good on earbuds or in your car (that means the 90% of us who don't own thousand dollar stereo systems).
A song that sounds dull or quiet will generally need to be played at higher volume, or with the iPod set for "sound check" (amped up to be at the same volume of the louder songs). This is no big deal at home, but having to use more volume can affect battery life in a mobile device, so sound quality counts. Some mp3s show the effects of indifferent mastering and sound flat (illegal downloads commonly have poor sound in most cases), while others seem to just jump out of the speakers.
That quality of "loudness" is important. It can impact battery life as pointed out earlier, and how enjoyable the music is.
An mp3 has less signal and digital data, so enough of the music has to come through to make you do whatever it is you do with music. That's not a criticism, of course...music has rarely come down to the market in it's purest form anyway, otherwise there'd no market for remastered albums. Whatever the artist or label intended you to hear, it was filtered to our ears, generally through small speakers, cheap stereos, and whatever the radio sounded like (and the old radio DJs could control that).
The silver lining is technology...today's listeners generally have standard access to equalizers, pre-set music mixes, better quality gear and music editing software. The aware consumer can always adjust things so the music is to their liking...
...if the music is good in the first place, but that's another subject.