I've downloaded public domain WAV files from the internet to generate one-shot drum sounds. Which ends up being a WAV file player cued by a piano key press. The format of the sample needs to match the supported format. Using any number of public domain (open source) audio converters can successfully convert.

I believe there's a thunder sample already in Montage (Waveform 2614 - SFX Kit key A5). Rain is probably there too (Rainloop 2628 - Jungle Kit) Bird may not be the type you need (Waveform 2624) because there's a distinct "tweet tweet" from the GM sound days. A stream sound is Waveform 2625. The seashore is 2621.

Since these are to fill in as "GM sounds" - they may not be up to par for a video production.

I would probably ask for permission before use - but whale sounds seem easy to find: https://www.nps.gov/glba/learn/nature/soundclips.htm

I see many royalty-free public-domain sound sites with many of the sounds you're asking for. Probably wouldn't take long to assemble from the sfx samples as you say.

This video is a quick overview of the process: https://www.youtube.com/watch?v=JXASiv3YfSE

It was generated when Montage firmware was in its initial release - but the general workflow applies. The only difference with the video (in the general case, maybe not your usage) I would have with the video is the sampling rate. I wouldn't have chosen 48kHz when given the choice. The native rate is 44.1kHz (16 or 24 bit) so I would use that. Since nature sounds are not very pitch sensitive - it wouldn't matter if you used 48kHz. But, in the general case, the pitch change from using 48kHz may matter. In the video, the sound is percussive and not pitch sensitive - so any pitch adjustments do not matter much like the nature sounds. The video seems to suggest there is absolutely no penalty for choosing the non-native format which is why I have any difference.

I have a number of audio-for-video projects that require atmospheric and ambient sounds – surf, wind, birds, rain, distant thunder, underwater, etc – this kind of thing. For example: audio to support video of a flyover of the Great Rift Valley, of moving through a rainforest, viewing a waterfall, and following whales underwater. I am wondering if I can achieve this from scratch-built voices on the Montage or possibly from Motif sound effects libraries.

I can certainly build this from sfx samples, but generating it in a flowing, interactive way doesn't always work well when using samples.

I believe the MOTION CONTROL SYNTHESIS ENGINE in the MONTAGE is ideal for this kind of work... assembling the data and loading it into the MONTAGE is the first task.

Then building the soundscape the way you want it to unfold will be the fun part... triggering samples can be one method, using the AWM2 architecture to construct the audio clips so that they behave in a way that creates an ever changing soundscape that is where you’ll have the most fun... using custom Arpeggios to trigger sounds, while others are triggered directly. Using the Motion Sequencer to add movement — not just in the stereo panorama (left-to-right) but close-to-far... using the AssignKnob/SuperKnob system to help create the illusion of traveling through an environment... Motion Sequences allow you create automated parameter changes; these can be Tempo sync’d, and can be manipulated in any number of creative ways.

There are Performances that show off this “proximity” effect... you get the sense of changing the listener’s position in relation to the sound... things like simultaneously changing Reverb Time, EQ, and Time Delay so that things have that feel of real depth — giving perception of moving through an environment.

Between samples and FM-X you should be able to create a rich palette of sound effects... and more importantly you will be able to create a way to manipulate the sound to match the visuals in a satisfying manner. Even to creating/synthesizing whale sounds. Yes, there is a “Thunder” Waveform... you can build an entire Part from that Waveform... pitched across the keyboard. Playing a varying number of keys gives a realistic feel to the “storm”. You can quickly emulate an approaching storm using the right amount of effects, notes, volume and creativity. Even moving it across the stereo field as it approaches.

Much of the fifteen years of Motif Libraries can serve as a resource...
There are some 6347 Waveforms resident in the MONTAGE Wave ROM... all Waveforms are “searchable” by Main and Sub Category.