Are true dynamic folders (NOT a "Smart Folder" SavedSearch) possible?
I have found a solution to create a dynamic "soft link" folder:

Preface:


$HOME is the result of the environmental variable HOME here. When you log in as user example_user, $HOME is then the path to the home folder of example_user: /Users/example_user.
If $HOME appears in the text below you have to replace it yourself with /Users/your_user_name/ (e.g. saving the launch agent). If $HOME appears in a code box below (e.g. in the shell script) it will automatically expanded to your home folder by the (bash) shell while executing the script.
mdimport -A|sed $'s/\t\t/;/g;s/(null)//g'|tr -d \' shows a lot of metadata attributes to choose from as search criteria (e.g. kMDItemAudioBitRate or kMDItemDurationSeconds)
mdls /path/to/file lists all metadata attributes of a chosen file


Creating a dynamic link folder:


Create a shell script samples.sh in $HOME/bin/sh/ with the following content:

#!/bin/bash

# Define folders
# Source folder (including files in subfolders)
MusicSamples=$HOME/Music/Samples
# Destination folder
DrumFoldr=$HOME/Desktop/Samples

# Remove orphaned soft links in the destination folder
find $DrumFoldr -type l -exec sh -c 'for x; do [ -e "$x" ] || rm "$x"; done' _ {} +

# Add soft links in destination folder depending on some criteria (here: BitRate and UserTag)
for File in $(mdfind -onlyin $MusicSamples 'kMDItemAudioBitRate &gt;= "44000"  &amp;&amp; _kMDItemUserTags = "Sample"')
do
    ln -s $File $DrumFoldr
done

# Remove soft links in the destination folder depending on some criteria (removing UserTag from original file)
for File in $(mdfind -onlyin $MusicSamples '_kMDItemUserTags != "Sample"')
do
    filename=$(basename "$File")
    rm $DrumFoldr/$filename
done


In the shell script define your music folder containing the sample files (in my example that's a Samples folder in the Music folder) and the "DrumFoldr" containing the soft links later (in my example that's a Samples folder on my Desktop).

Now choose some Spotlight search criteria (in my example I have chosen the bit rate and a user defined tag "Sample" - you can define user tags by right-clicking a file > Tags... and adding a tag by just writing something. The user defined tag is then available for other files and folders.



The above shell script will add only files with a sample rate of 44000 or higher and (this is a logical and!) with the user tag "Sample" as soft link to the DrumFoldr folder. So the destination folder will not contain links to files without the Sample tag but with a sample rate of 96k or files with the Sample tag but without a kMDItemAudioBitRate (like txt files or pdfs). It also does some cleansing in the DrumFoldr folder.

BTW: the music length can be searched/determined by using the key kMDItemDurationSeconds: kMDItemDurationSeconds &lt;= "2".
Create a file usr.samplesync.plist in $HOME/Library/LaunchAgents (that means: /Users/&lt;username&gt;/Library/LaunchAgents/) with the content:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;usr.samplesync&lt;/string&gt;
    &lt;key&gt;ProgramArguments&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;/bin/bash&lt;/string&gt;
        &lt;string&gt;/Users/&lt;username&gt;/bin/sh/samples.sh&lt;/string&gt;
    &lt;/array&gt;
    &lt;key&gt;RunAtLoad&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;StartInterval&lt;/key&gt;
    &lt;integer&gt;10&lt;/integer&gt;
&lt;/dict&gt;
&lt;/plist&gt;


Replace &lt;username&gt; by your short user name. The shell script will be executed every 10 seconds. You can increase the interval depending on your needs and environment to higher values (e.g. 60 = 1 minute or 600 = 10 minutes)
Load the launch agent with:

launchctl load $HOME/Library/LaunchAgents/usr.samplesync.plist





If you want to save the shell script somewhere else you have to apply the path in the plist accordingly.



Caveats: 


the original sample files mustn't contain spaces in their paths or file names (I try to solve this restriction later)
the original sample files mustn't be moved or the removing of the user tag won't delete the soft link


Things to improve:


don't create a soft link if a proper one already exists