I have combined 12 MP3's to one single MP3. When I play the MP3 I can hear slight gaps between each song.

I opened the MP3 file in MP3DirectCut and can see the gaps between each song. Is there software out there that automatically detects these gaps and removes them, without losing quality in sound?

I don't think so. These are intrinsic to MP3 (fixed frame size and zero-padding) though the LAME header allows a fix upon decoding (by providing encoder delay and padding values to any decoder that supports it - i.e LAME itself, foobar2000 and I think, iPod/iTunes).

Without transcoding or re-ripping from the original, your best bet is probably to delete the gaps manually in mp3DirectCut or perhaps try its Edit/Pause Detection function with extremely short minimum pause and very low threshold (though you risk deleting intentional gaps). The timing won't be perfect (can only cut on frame boundaries) but is likely to be virtually indistinguishable from the original from my limited experimentation in the past.

MP3DirectCut is awful for trying to zoom in close enough to remove the gaps.

I can do it without a problem in Audacity as it allows you to zoom up as close as possible, but the problem with Audacity is that you can't save your edited MP3's without further re-encoding them.

Does anyone know of a good mp3 editor that allows you to zoom in close and can edit MP3's losslessly?

The point with mp3DirectCut is that it can only cut whole mp3 frames (1152 samples, about 26 ms), unlike Audacity, which can cut individual samples.

Thus, its maximum zoom shows only brief gaps around frame boundaries and they're not always exactly zero, because mp3DirectCut seems to use Global Gain as its display of approximate loudness, rather than actual waveform size.

What I've sometimes done is use track length information to get to roughly the right place (type in the time) or loaded a CUEsheet (e.g. download from Cuesheet Heaven website). The track divisions are usually very close indeed to the right places, and you can jump to the next marker easily. If you'd used mp3DirectCut to join them in the first place, you'd also have track markers in the right places, as I recall.

I then make a selection and audition it as a cut (play the 2 seconds before and the 2 seconds after) to see how seamless it sounds. This works for me.

mp3DirectCut has a few rivals with similar features, but I've not tried cutting track gaps with them.

You might wish to bear in mind pcutmp3 for some uses as it correctly accounts for bit-reservoir unlike most or all other mp3 cutting programs. You might be able to use it to strip away the first and last frames before stitching the track MP3s together (or whatever trimming seems right) and might then find the trimmed-then-joined file acceptable without further changes.

The very limited graph and ability to quickly prelisten without launching a media player and seeking is very useful. Try working with AC-3/DTS/AAC with just a hex editor. To workaround the bit reservoir problem Mp3DirectCut can team up with MP3Packer to first inflate the streams to 320 kBit/s with minimal reservoir, then after the edits compress the stream back to minimal size.

However, no mp3 editor will restore an improperly ripped gapless album.

Great point about MP3Packer (or WinMP3Packer) to pad it out to 320 kbps CBR and virtually eliminate bit reservoir usage before cutting. I fully endorse that method as about the best you'll get if you must stay with mp3 (e.g. for player compatibility)

You could do better in gap-removal if you decode to lossless to cut the gaps (esp if your MP3s have LAME gapless info) and never re-encode to MP3. I'd then suggest using a .fromMP3.wav type of double extension and a comment tag field to indicate clearly what you've done. If you want to save some bitrate afterwards with minimal transcoding artifacts, you could use lossyWAV (followed by FLAC, WV or TAK) or wavpack lossy.

Are there any MP3 editors with waveform display that allow you to edit files losslessly?

I haven't seen any. Given that they cannot work losslessly at any time interval smaller than MP3 frame boundaries, the best they do is usually to indicate a pseudo-waveform-view envelope by plotting the value of the global gain for each frame (as does mp3DirectCut).

Would it not be possible to decode the MP3 to PCM, and display this waveform over the MP3 frames? You of course would not be able to make sample-by-sample edits, but at least you could see the actual waveform instead of just the gain.

Whether there are currently tools that do this is another matter.