@Dileep As a personal comment regarding your requirements for sound, I’ve worked professionally in animation, compositing and editing departments and after experiencing the full script to screen workflow in a few productions, to be honest Pencil2D is NOT an appropriate tool for video editing and by extension animatic /leica reel authoring.
Of course I’m not saying most of your requests related to sound won’t happen. That is not the case and actually a few of them should hopefully still occur down the line once they are reviewed in the context of the application.
While we want Pencil2D to have suitable capabilities to handle sound files so you can work with dialogue and sparse sound effects PER scene / file, if you are doing animatics for a living please consider using a video editor instead, particularly if you’re handling more than 20 sound tracks at all times or if your’re handling broadcast files with an average length of 8 to 24 minutes and above.
Note that i’m only suggesting this because it is highly probable that Pencil2D will never go the route of becoming a full-fledged video editor in terms of capacity to handle super long files with multiple tracks, as it is primarily thought and developed as an animation tool.
Aside from that I’m a firm proponent of using specialized tools for specialized workflows. I personally edit all my animatics using video editing software like Sony Vegas, Adobe Premiere and more recently KDenlive.
Having a few sounds should in not way be a problem for Pencil2D in the future, but managing tv show / feature film levels of intricate sound layering in Pencil2d, or even in Flash, Toonboom Harmony or any other animation program is quite frankly bad practice from a production management perspective.
With that said here’s a list of several free / open source video editors that are robust enough to handle the work you’re describing and which should be perfect companions for your work with Pencil2D or Flash 
–Open-Source–
- OpenShot https://openshot.org
- KDEnlive [https://kdenlive.org/en/]
- Shotcut [https://shotcut.org]
- Olive (Beta) [https://olivevideoeditor.org]
–Free–
- Lightworks [https://lwks.com]
- Hitfilm Express [https://fxhome.com/hitfilm-express]
- Davinci Resolve [https://www.blackmagicdesign.com/products/davinciresolve/]